Teach dag combine to match halfword byteswap patterns.

1. (((x) & 0xFF00) >> 8) | (((x) & 0x00FF) << 8)
   => (bswap x) >> 16
2. ((x&0xff)<<8)|((x&0xff00)>>8)|((x&0xff000000)>>8)|((x&0x00ff0000)<<8))
   => (rotl (bswap x) 16)

This allows us to eliminate most of the def : Pat patterns for ARM rev16
revsh instructions. It catches many more cases for ARM and x86.

rdar://9609108

llvm-svn: 133503