[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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311639 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed