[X86AsmParser] Refactoring, (almost) NFC.

Some refactoring to X86AsmParser, mostly regarding the way rewrites are conducted.
Mainly, we try to concentrate all the rewrite effort under one hood, so it'll hopefully be less of a mess and easier to maintain and understand.
naturally, some frontend tests were affected: D36794

Differential Revision: https://reviews.llvm.org/D36793

llvm-svn: 311639