Implement default legalization of rotates: either in terms of the rotation in the opposite direction (if legal), or in terms of shifts and ors.
Implement generating of rotate instructions for Hexagon. Hexagon only supports rotates by an immediate value, so implement custom lowering of ROTL/ROTR on Hexagon. If a rotate is not legal, use the default expansion.
This might be useful as general helper function - add it to the DAGCombiner class for reuse?