[X86] For 32-bit targets, emit two-byte NOP when possible
In order to support hot-patching, we need to make sure the first emitted instruction in a function is a two-byte+ op. This is already the case on x86_64, which seems to always emit two-byte+ ops. However on 32-bit targets this wasn't the case.
PATCHABLE_OP now lowers to a XCHG AX, AX, (66 90) like MSVC does. However when targetting pentium3 (/arch:SSE) or i386 (/arch:IA32) targets, we generate MOV EDI,EDI (8B FF) like MSVC does. This is for compatiblity reasons with older tools that rely on this two byte pattern.
Differential Revision: https://reviews.llvm.org/D81301