ARM: use natural LLVM IR for vshll instructions

Similarly to the vshrn instructions, these are simple zext/sext + trunc
operations. Using normal LLVM IR should allow for better code, and more sharing
with the AArch64 backend.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201093 91177308-0d34-0410-b5e6-96231b3b80d8