[MC] Fix Intel Operand assembly parsing for .set ids

Recommitting after fixing overaggressive fastpath return in parsing.

Fix intel syntax special case identifier operands that refer to a constant
(e.g. .set <ID> n) to be interpreted as immediate not memory in parsing.

Associated commit to fix clang test commited shortly.

Reviewers: rnk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D22585

llvm-svn: 277489