[X86] Add a private member function determinePaddingPrefix for X86AsmBackend

Summary: X86 can reduce the bytes of NOP by padding instructions with prefixes to get a better peformance in some cases. So a private member function `determinePaddingPrefix` is added to determine which prefix is the most suitable.

Reviewers: annita.zhang, reames, MaskRay, craig.topper, LuoYuanke, jyknight

Reviewed By: reames

Subscribers: llvm-commits, dexonsmith, hiraditya

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75357