ARM: implement vshrn NEON intrinsic in terms of shr/trunc

Now the backend supports the natural LLVM IR, we can shamelessly steal the
AArch64 front-end code to implement the vshrn intrinsic on 32-bit ARM.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201086 91177308-0d34-0410-b5e6-96231b3b80d8