ARM: constrain register-class in fast-isel

The tests were no longer using fast-isel at all (MachO needs an "ios" rather
than "darwin" triple at the moment and Linux needs ARM mode). Once that was
corrected, the verifier complained about a t2ADDri created for the alloca.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197046 91177308-0d34-0410-b5e6-96231b3b80d8