[llvm] [LegalizeDAG] Optimize CodeGen for `ISD::CTLZ_ZERO_UNDEF` (PR #83039)

[Matt Arsenault via llvm-commits]

================
@@ -2458,13 +2458,20 @@ LegalizerHelper::widenScalar(MachineInstr &MI, unsigned TypeIdx, LLT WideTy) {
       NewOpc = TargetOpcode::G_CTTZ_ZERO_UNDEF;
     }
 
+    unsigned SizeDiff = WideTy.getSizeInBits() - CurTy.getSizeInBits();
+
+    if (MI.getOpcode() == TargetOpcode::G_CTLZ_ZERO_UNDEF) {
+      // An optimization where the result is the CTLZ after the left shift by
+      // (Difference in widety and current ty).
----------------
arsenm wrote:

DAG style comment would be helpful 

https://github.com/llvm/llvm-project/pull/83039