Remove rather oddly merged logic from AArch64 commit.

We seem to have ended up with both an inlined check of permitted NEON base
types and a call to a function.

Since the outer if was (I believe) strictly weaker than the one in the
function, there's no actual user-visible behaviour change, so no tests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187652 91177308-0d34-0410-b5e6-96231b3b80d8