ARM: move vshll NEON implementation to common code

Now that both ARM backends use the same implementation for vshll operations,
the code can be shared. This is also a necessary LLVM/Clang interface update.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201094 91177308-0d34-0410-b5e6-96231b3b80d8