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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166592 91177308-0d34-0410-b5e6-96231b3b80d8