MJIT: Remove the code to optimize shape transition

because this code crashes on railsbench. I'll try adding a repro for it
later, but I don't know shapes enough to craft it right away.

lib/mjit/compiler.rb

@@ -376,19 +376,9 @@ def compile_ivar(insn_name, stack_size, pos, status, operands, body)
         src << "    VALUE obj = GET_SELF();\n"
         # JIT: cache hit path of vm_getivar/vm_setivar, or cancel JIT (recompile it with exivar)
         if insn_name == :setinstancevariable
-          src << "    const shape_id_t source_shape_id = (shape_id_t)#{source_shape_id};\n"
           src << "    const uint32_t index = #{attr_index - 1};\n"
           src << "    const shape_id_t dest_shape_id = (shape_id_t)#{dest_shape_id};\n"
-          src << "    if (source_shape_id == ROBJECT_SHAPE_ID(obj) && \n"
-          src << "        dest_shape_id != ROBJECT_SHAPE_ID(obj)) {\n"
-          # Conditionally generate a capacity change if there is one
-          # between the destination and the parent IV set
-          src << "        rb_ensure_iv_list_size(obj, ROBJECT_IV_CAPACITY(obj), #{capa});\n" if capa
-          src << "        ROBJECT_SET_SHAPE_ID(obj, dest_shape_id);\n"
-          src << "        VALUE *ptr = ROBJECT_IVPTR(obj);\n"
-          src << "        RB_OBJ_WRITE(obj, &ptr[index], stack[#{stack_size - 1}]);\n"
-          src << "    }\n"
-          src << "    else if (dest_shape_id == ROBJECT_SHAPE_ID(obj)) {\n"
+          src << "    if (dest_shape_id == ROBJECT_SHAPE_ID(obj)) {\n"
           src << "        VALUE *ptr = ROBJECT_IVPTR(obj);\n"
           src << "        RB_OBJ_WRITE(obj, &ptr[index], stack[#{stack_size - 1}]);\n"
           src << "    }\n"

test/ruby/test_mjit.rb

@@ -856,7 +856,7 @@ def bar
   end
 
   def test_inlined_undefined_ivar
-    assert_eval_with_jit("#{<<~"begin;"}\n#{<<~"end;"}", stdout: "bbb", success_count: 2, call_threshold: 2)
+    assert_eval_with_jit("#{<<~"begin;"}\n#{<<~"end;"}", stdout: "bbb", success_count: 5, call_threshold: 2)
     begin;
       class Foo
         def initialize