[DAGCombiner] matchRotateSub - handle shift amount truncation

Under certain circumstances we'll end up in the position where the negated shift amount will get truncated to the type specified getScalarShiftAmountTy(), so we need to test for a truncated version of the shift amount as well.

This allows us to remove half of the remaining patterns tested for by X86ISelLowering's combineOrShiftToFunnelShift.