[llvm] [SelectionDAG] Look through ISD::FREEZE in SimplifyDemanded checks (PR #200931)

[Simon Pilgrim via llvm-commits]

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@@ -714,8 +714,10 @@ SDValue TargetLowering::SimplifyMultipleUseDemandedBits(
   if (Depth >= SelectionDAG::MaxRecursionDepth)
     return SDValue();
 
-  // Ignore UNDEFs.
-  if (Op.isUndef())
+  // Ignore undef/poison, including frozen undef/poison. Replacing a freeze of
+  // undef/poison with another undef node is unnecessary and can fight with
+  // freeze sinking.
+  if (peekThroughFreeze(Op).isUndef())
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RKSimon wrote:

it would be better if you do a pr fr each of these separately, each with their own test coverage

https://github.com/llvm/llvm-project/pull/200931