[X86AsmParser] Refactor AsmRewrite constructors, NFCI
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@311666 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm