[SelectionDAG] Support scalable splats in U(ADD|SUB)SAT combines

This patch builds on top of D106575 in which scalable-vector splats were
supported in `ISD::matchBinaryPredicate`. It teaches the DAGCombiner how
to perform a variety of the pre-existing saturating add/sub combines on
scalable-vector types.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D106652