[ms-inline asm] Support offsets after segment registers

Summary: MASM let's you do stuff like 'MOV FS:20, EAX' and 'MOV EAX, FS:20'

Reviewers: craig.topper, rnk

Reviewed By: rnk

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1470