Add support bswap16 to/from memory compiling to rev16 on ARM/Thumb

The current patterns for REV16 misses mostn __builtin_bswap16() due to
legalization promoting the operands to from load/stores toi32s and then
truncing/extending them. This patch adds new patterns that catch the resultant
DAGs and codegens them to rev16 instructions. Tests included.

rdar://15353652

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208620 91177308-0d34-0410-b5e6-96231b3b80d8