[llvm] [DAGCombiner] Fold `sub nuw C, x` -> `xor x, C` when C is a mask (PR #192692)

[Matt Arsenault via llvm-commits]

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@@ -4549,6 +4549,16 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
     }
   }
 
+  // sub nuw C, x --> xor x, C when C is a mask (2^k - 1)
+  if (ConstantSDNode *C0 = isConstOrConstSplat(N0)) {
+    if (N->getFlags().hasNoUnsignedWrap()) {
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arsenm wrote:

There are other isConstOrConstSplat blocks already here, should move them together 

https://github.com/llvm/llvm-project/pull/192692