Convert a few more getAs cases to castAs where we immediately call a member function.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@163684 91177308-0d34-0410-b5e6-96231b3b80d8
clang: add support for neon-vfp instructions

 rebased for cm10.1-staging
Change-Id: Iba9b39b286f97117f1e93dcc5d234a44f02a46de