Tests: use CHECK-LABEL to help debugging failures

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210906 91177308-0d34-0410-b5e6-96231b3b80d8
IR-change: cmpxchg operations now return { iN, i1 }.

This is a minimal fix for clang. I'll soon add support for generating
weak variants when requested, but that's not really necessary for the
LLVM change in isolation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210907 91177308-0d34-0410-b5e6-96231b3b80d8