[SelectionDAG] ComputeNumSignBits - cleanup ROTL/ROTR wrapping to match DAGCombine etc.

Use RotAmt.urem(VTBits) instead of AND(RotAmt, VTBits - 1)

TBH I don't expect non-power-of-2 types to be created, but it makes the logic clearer and matches what we do in other rotation combines.

llvm-svn: 313245