Fix TableGen subtarget-emitter to handle A9/Swift.

A9 uses itinerary classes, Swift uses RW lists. This tripped some
verification when we're expanding variants. I had to refine the
verification a bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178357 91177308-0d34-0410-b5e6-96231b3b80d8