[ms-inline asm] Create a register operand, rather than a memory operand when we

see the offsetof operator.  Previously, we were matching something like MOVrm
in the front-end and later matching MOVrr in the back-end.  This change makes
things more consistent.  It also fixes cases where we can't match against a 
memory operand as the source (test cases coming).
Part of rdar://12470317

llvm-svn: 166592