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

Authored by frasercrmck on Jul 23 2021, 3:13 AM.

Description

[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

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