[ia32] Tentative fix for invalid codegen on ia32

It looks like codegen on ia32 may sometimes miss a byte during
emit_arith. Our working theory is that this happens because `length`
in emit_operand, for whatever reason, is 0 and we thus do not advance
`pc_` correctly. The tentative fix is to use EMIT to increment the pc_
on each single write instead of relying on `length`.

Bug: v8:9774
Change-Id: I522eef96481f44f57628f914c9c170f1644ec47a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1824941
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63978}

src/codegen/ia32/assembler-ia32.cc

@@ -3182,11 +3182,10 @@ void Assembler::emit_operand(int code, Operand adr) {
   DCHECK_GT(length, 0);
 
   // Emit updated ModRM byte containing the given register.
-  pc_[0] = (adr.buf_[0] & ~0x38) | (code << 3);
+  EMIT((adr.buf_[0] & ~0x38) | (code << 3));
 
   // Emit the rest of the encoded operand.
-  for (unsigned i = 1; i < length; i++) pc_[i] = adr.buf_[i];
-  pc_ += length;
+  for (unsigned i = 1; i < length; i++) EMIT(adr.buf_[i]);
 
   // Emit relocation information if necessary.
   if (length >= sizeof(int32_t) && !RelocInfo::IsNone(adr.rmode_)) {