ARM: add natural patterns for vaddhl and vsubhl.

These instructions aren't particularly complicated and it's well worth having
patterns for some reasonably useful LLVM IR that will match them. Soon we
should be able to switch Clang over to producing this natural version.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189335 91177308-0d34-0410-b5e6-96231b3b80d8