ARM: use LLVM IR to represent the vshrn operation

vshrn is just the combination of a right shift and a truncate (and the limits
on the immediate value actually mean the signedness of the shift doesn't
matter). Using that representation allows us to get rid of an ARM-specific
intrinsic, share more code with AArch64 and hopefully get better code out of
the mid-end optimisers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201085 91177308-0d34-0410-b5e6-96231b3b80d8