[DAG] PromoteIntRes_ADDSUBSHLSAT - use promoted ISD::USUBSAT directly

As discussed on D96413, as long as the promoted bits of the args are zero we can use the basic ISD::USUBSAT pattern directly, without the shifting like we do for other ops.

I think something similar should be possible for ISD::UADDSAT as well, which I'll look at later.

Also, create a ISD::USUBSAT node directly - this will be expanded back by the legalizer later on if necessary.

Differential Revision: https://reviews.llvm.org/D96622