Avoid dup and pop in conditional assignments

[HParker]

Remove some extra dup and pop generated in ||= and &&=

ruby --dump=insns -e 'x ||= 1; nil'

before:

== disasm: #<ISeq:<main>@-e:1 (1,0)-(1,12)> (catch: FALSE)
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] x@0
0000 getlocal_WC_0                          x@0                       (   1)[Li]
0002 dup
0003 branchif                               10
0005 pop
0006 putobject_INT2FIX_1_
0007 dup
0008 setlocal_WC_0                          x@0
0010 pop
0011 putnil
0012 leave

after:

== disasm: #<ISeq:<main>@-e:1 (1,0)-(1,12)> (catch: false)
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] x@0
0000 getlocal_WC_0                          x@0                       (   1)[Li]
0002 branchif                               7
0004 putobject_INT2FIX_1_
0005 setlocal_WC_0                          x@0
0007 putnil
0008 leave

ruby --dump=insns -e 'x.y ||= 1; nil'

before:

== disasm: #<ISeq:<main>@-e:1 (1,0)-(1,14)> (catch: FALSE)
0000 putself                                                          (   1)[Li]
0001 opt_send_without_block                 <calldata!mid:x, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0003 dup
0004 opt_send_without_block                 <calldata!mid:y, argc:0, ARGS_SIMPLE>
0006 dup
0007 branchif                               18
0009 pop
0010 putobject_INT2FIX_1_
0011 swap
0012 topn                                   1
0014 opt_send_without_block                 <calldata!mid:y=, argc:1, ARGS_SIMPLE>
0016 jump                                   19
0018 swap
0019 pop
0020 pop
0021 putnil
0022 leave

after:

== disasm: #<ISeq:<main>@-e:1 (1,0)-(1,14)> (catch: false)
0000 putself                                                          (   1)[Li]
0001 opt_send_without_block                 <calldata!mid:x, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0003 dup
0004 opt_send_without_block                 <calldata!mid:y, argc:0, ARGS_SIMPLE>
0006 branchif                               11
0008 putobject_INT2FIX_1_
0009 opt_send_without_block                 <calldata!mid:y=, argc:1, ARGS_SIMPLE>
0011 pop
0012 putnil
0013 leave

compare-ruby: ruby 3.2.0dev (2022-04-04T13:27:14Z master ea9c09a92c) [x86_64-darwin21]
built-ruby: ruby 3.2.0dev (2022-09-21T21:12:45Z avoid-dup-and-pop-.. da183a284c) [x86_64-darwin21]
# Iteration per second (i/s)

|                  |compare-ruby|built-ruby|
|:-----------------|-----------:|---------:|
|vm_lvar_cond_set  |     63.841M|  109.491M|
|                  |           -|     1.72x|