[X86AsmParser] Refactor AsmRewrite constructors, NFCI

Summary:
This is a follow-up of https://reviews.llvm.org/D37105, where a slight refactoring
of the constructors of AsmRewrite is proposed.

Reviewers: coby

Reviewed By: coby

Differential Revision: https://reviews.llvm.org/D37110

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311666 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed