[MC][X86] Enhance X86 Register expression handling to more closely match GCC.

Allow the comparison of x86 registers in the evaluation of assembler
directives. This generalizes and simplifies the extension from r334022
to catch another case found in the Linux kernel.

Reviewers: rnk, void

Reviewed By: rnk

Subscribers: hiraditya, nickdesaulniers, llvm-commits

Differential Revision: https://reviews.llvm.org/D50795

llvm-svn: 339895