Assert that the last operand is actually EFLAGS

This is another follow-up to r197503, after a post-commit review by
Andy.

<rdar://problem/15627766>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197520 91177308-0d34-0410-b5e6-96231b3b80d8