[llvm] [DAGCombine] Fold `ctlz_zero_undef(X << C) -> ctlz_zero_undef(X) - C` (PR #100932)

[Matt Arsenault via llvm-commits]

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@@ -11133,6 +11133,18 @@ SDValue DAGCombiner::visitCTLZ_ZERO_UNDEF(SDNode *N) {
   if (SDValue C =
           DAG.FoldConstantArithmetic(ISD::CTLZ_ZERO_UNDEF, DL, VT, {N0}))
     return C;
+
+  // Fold ctlz_zero_undef(X << C) --> ctlz_zero_undef(X) - C
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arsenm wrote:

It's not obvious to me that it's better. it's just trading a shift for an add or sub?

https://github.com/llvm/llvm-project/pull/100932