IRGen: __c11/__atomic compare-and-exchange should respect the standard

Summary:
'Expected' should only be modified if the operation fails.

This fixes PR18899.

Reviewers: chandlerc, rsmith, rjmccall

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2922

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203493 91177308-0d34-0410-b5e6-96231b3b80d8