[X86] Fix register resizings for inline assembly register operands.

When replacing a named register input to the appropriately sized
sub/super-register. In the case of a 64-bit value being assigned to a
register in 32-bit mode, match GCC's assignment.

Reviewers: eli.friedman, craig.topper

Subscribers: nickdesaulniers, llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D51502

llvm-svn: 342175