[X86] Fix incomplete handling of register-assigned variables in parsing.

Teach x86 assembly operand parsing to distinguish between assembler
variable assigned to named registers and those assigned to immediate
values.

Reviewers: rnk, nickdesaulniers, void

Subscribers: hiraditya, jyknight, llvm-commits

Differential Revision: https://reviews.llvm.org/D56287

llvm-svn: 350966