Teach x86 unwinder about POP ECX being optional optimization

[filipnavara]

Fixes #115120

The same pattern already exists in the unwinder elsewhere:

runtime/src/coreclr/vm/gc_unwind_x86.inl

Lines 2446 to 2449 in 2f8f6e7

	if (epilogBase[0] == X86_INSTR_POP_ECX)
	{
	// We may use "POP ecx" for doing "ADD ESP, 4",
	// or we may not (in the case of JMP epilogs)

[filipnavara]

offset is always 0 at this point, so epilogBase[0] and epilogBase[offset] is the same thing

[janvorli]

I think it would be better to use the offset even though it is zero. All the remaining code uses offset, even in the code inside of this if, so using 0 here might be confusing for the reader.

[filipnavara]

I changed both of the places with the pattern. Optimizing compiler should propagate the constant anyway.

[dotnet-policy-service]

Tagging subscribers to this area: @mangod9
See info in area-owners.md if you want to be subscribed.

[janvorli]

It looks good modulo the comment.

[janvorli]

I think it would be better to use the offset even though it is zero. All the remaining code uses offset, even in the code inside of this if, so using 0 here might be confusing for the reader.

[janvorli]

LGTM, thank you!