ARM: Fix incorrect pack pattern

A "pkhtb x, x, y asr #num" uses the lower 16 bits of "y asr #num" and packs them
in the bottom half of "x". An arithmetic and logic shift are only equivalent in
this context if the shift amount is 16. We would be shifting in ones into the
bottom 16bits instead of zeros if "y" is negative.

radar://14338767

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185712 91177308-0d34-0410-b5e6-96231b3b80d8
ARM: Add a pack pattern for matching arithmetic shift right

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185714 91177308-0d34-0410-b5e6-96231b3b80d8