Preserve early clobber flag when using named registers in inline assembly.

Summary:
Named registers with the constraint "=&r" currently lose the early clobber flag
and turn into "=r" when converted to LLVM-IR. This patch correctly passes it on.

Reviewers: atanasyan

Reviewed By: atanasyan

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7346

llvm-svn: 228143