Attempt to pacify buildbots after r280217

These clang tests check diagnostics from the backend by giving it an unvectorizable loop. This loop is now vectorized :/

Make it really unvectorizable by making it unprofitable to ifconvert.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@280220 91177308-0d34-0410-b5e6-96231b3b80d8