[LegalizeDAG] Expand i16 bswap directly to a rotate by 8 instead of relying on DAG combine.

An i16 bswap can be implemented with an i16 rotate by 8. We previously emitted
a shift and OR sequence that DAG combine should be able to turn back into
rotate. But we might as well go there directly. If rotate isn't legal,
LegalizeDAG should further legalize it to either the opposite rotate, or the
shift and OR pattern.

I don't know of any way to get the existing DAG combine reliance to fail. So
I don't know any way to add new tests for this that wouldn't have worked
previously.

llvm-svn: 356860