[DAGCombiner] rot i16 X, 8 --> bswap X

We have this generic transform in IR (instcombine),
but as shown in PR41098:
http://bugs.llvm.org/PR41098
...the pattern may emerge in codegen too.

x86 has a potential refinement/reversal opportunity here,
but that should come later or needs a target hook to
avoid the transform. Converting to bswap is the more
specific form, so we should use it if it is available.