Call rb_ivar_get and rb_ivar_set instead of exiting for many ivars

Previously, when we have a lot of ivars defined, we would exit via
`jit_chain_guard` for megamorphic ivars. Now if we have more than the
max depth of ivars we can call `rb_ivar_get` and `rb_ivar_set`
respectively.

Using the following script:

```ruby
class A
  def initialize
    @A = 1
  end

  def a
    @A
  end
end

N = 30
N.times do |i|
  eval <<-eorb
class A#{i} < A
  def initialize
    @A#{i} = 1
    super
  end
end
  eorb
end

klasses = N.times.map { Object.const_get(:"A#{_1}") }

1000.times do
  klasses.each do |k|
    k.new.a
  end
end
```

Exits before this change show exits for `getinstancevariable` and
`setinstancevariable`:

```
***YJIT: Printing YJIT statistics on exit***
method call exit reasons:
    klass_megamorphic:     24,975 (100.0%)
invokeblock exit reasons:
    (all relevant counters are zero)
invokesuper exit reasons:
    (all relevant counters are zero)
leave exit reasons:
    interp_return:     26,948 (100.0%)
     se_interrupt:          1 ( 0.0%)
getblockparamproxy exit reasons:
    (all relevant counters are zero)
getinstancevariable exit reasons:
    megamorphic:     13,986 (100.0%)
setinstancevariable exit reasons:
    megamorphic:     19,980 (100.0%)
opt_aref exit reasons:
    (all relevant counters are zero)
expandarray exit reasons:
    (all relevant counters are zero)
opt_getinlinecache exit reasons:
    (all relevant counters are zero)
invalidation reasons:
    (all relevant counters are zero)
num_send:                    155,823
num_send_known_class:              0 ( 0.0%)
num_send_polymorphic:        119,880 (76.9%)
bindings_allocations:              0
bindings_set:                      0
compiled_iseq_count:              36
compiled_block_count:            158
compiled_branch_count:           240
block_next_count:                 10
defer_count:                      70
freed_iseq_count:                  0
invalidation_count:                0
constant_state_bumps:              0
inline_code_size:             29,216
outlined_code_size:           27,948
freed_code_size:                   0
code_region_size:             65,536
live_context_size:             8,322
live_context_count:              219
live_page_count:                   4
freed_page_count:                  0
code_gc_count:                     0
num_gc_obj_refs:                 130
object_shape_count:              295
side_exit_count:              58,942
total_exit_count:             85,890
yjit_insns_count:          1,023,581
avg_len_in_yjit:                11.2
Top-4 most frequent exit ops (100.0% of exits):
    opt_send_without_block:     24,975 (42.4%)
       setinstancevariable:     19,980 (33.9%)
       getinstancevariable:     13,986 (23.7%)
		     leave:          1 ( 0.0%)
```

Exits after this change show we have no exits for `getinstanevariable`
and `setinstancevariable`.

```
***YJIT: Printing YJIT statistics on exit***
method call exit reasons:
    klass_megamorphic:     24,975 (100.0%)
invokeblock exit reasons:
    (all relevant counters are zero)
invokesuper exit reasons:
    (all relevant counters are zero)
leave exit reasons:
    interp_return:     60,912 (100.0%)
     se_interrupt:          3 ( 0.0%)
getblockparamproxy exit reasons:
    (all relevant counters are zero)
getinstancevariable exit reasons:
    (all relevant counters are zero)
setinstancevariable exit reasons:
    (all relevant counters are zero)
opt_aref exit reasons:
    (all relevant counters are zero)
expandarray exit reasons:
    (all relevant counters are zero)
opt_getinlinecache exit reasons:
    (all relevant counters are zero)
invalidation reasons:
    (all relevant counters are zero)
num_send:                    155,823
num_send_known_class:              0 ( 0.0%)
num_send_polymorphic:        119,880 (76.9%)
bindings_allocations:              0
bindings_set:                      0
compiled_iseq_count:              36
compiled_block_count:            179
compiled_branch_count:           240
block_next_count:                 11
defer_count:                      70
freed_iseq_count:                  0
invalidation_count:                0
constant_state_bumps:              0
inline_code_size:             31,032
outlined_code_size:           29,708
freed_code_size:                   0
code_region_size:             65,536
live_context_size:             8,360
live_context_count:              220
live_page_count:                   4
freed_page_count:                  0
code_gc_count:                     0
num_gc_obj_refs:                 130
object_shape_count:              295
side_exit_count:              24,978
total_exit_count:             85,890
yjit_insns_count:          1,076,966
avg_len_in_yjit:                12.2
Top-2 most frequent exit ops (100.0% of exits):
    opt_send_without_block:     24,975 (100.0%)
		     leave:          3 ( 0.0%)
```

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

yjit/src/codegen.rs

@@ -1951,7 +1951,7 @@ fn gen_get_ivar(
     //       Eventually, we can encode whether an object is T_OBJECT or not
     //       inside object shapes.
     // too-complex shapes can't use index access, so we use rb_ivar_get for them too.
-    if !receiver_t_object || uses_custom_allocator || comptime_receiver.shape_too_complex() {
+    if !receiver_t_object || uses_custom_allocator || comptime_receiver.shape_too_complex() || (ctx.get_chain_depth() as i32) >= max_chain_depth {
         // General case. Call rb_ivar_get().
         // VALUE rb_ivar_get(VALUE obj, ID id)
         asm.comment("call rb_ivar_get()");
@@ -2177,7 +2177,7 @@ fn gen_setinstancevariable(
     // If the receiver isn't a T_OBJECT, or uses a custom allocator,
     // then just write out the IV write as a function call.
     // too-complex shapes can't use index access, so we use rb_ivar_get for them too.
-    if !receiver_t_object || uses_custom_allocator || comptime_receiver.shape_too_complex() {
+    if !receiver_t_object || uses_custom_allocator || comptime_receiver.shape_too_complex() || (ctx.get_chain_depth() as i32) >= SET_IVAR_MAX_DEPTH {
         asm.comment("call rb_vm_setinstancevariable()");
 
         let ic = jit_get_arg(jit, 1).as_u64(); // type IVC