ARM: Fix incorrect pack pattern for thumb2

Propagate the fix from r185712 to Thumb2 codegen as well. Original
commit message applies here as well:

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.

rdar://14338767