[MC][X86] Allow assembler variable assignment to register name.

Summary:
Allow extended parsing of variable assembler assignment syntax and modify X86 to permit
VAR = register assignment. As we emit these as .set directives when possible, we inline
such expressions in output assembly.

Fixes PR37425.

Reviewers: rnk, void, echristo

Reviewed By: rnk

Subscribers: nickdesaulniers, llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D47545

llvm-svn: 334022