[llvm] [VPlan] Verify dominance for incoming values of phi-like recipes. (PR #124838)

[via llvm-commits]

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@@ -220,12 +219,28 @@ bool VPlanVerifier::verifyVPBasicBlock(const VPBasicBlock *VPBB) {
 
       for (const VPUser *U : V->users()) {
         auto *UI = cast<VPRecipeBase>(U);
-        // TODO: check dominance of incoming values for phis properly.
-        if (!UI ||
-            isa<VPHeaderPHIRecipe, VPWidenPHIRecipe, VPPredInstPHIRecipe,
-                VPIRPhi>(UI) ||
-            (isa<VPInstruction>(UI) &&
-             cast<VPInstruction>(UI)->getOpcode() == Instruction::PHI))
+        if (auto *Phi = dyn_cast<VPPhiAccessors>(UI)) {
+          for (unsigned Idx = 0; Idx != Phi->getNumIncoming(); ++Idx) {
+            VPValue *IncVPV = Phi->getIncomingValue(Idx);
+            const VPBasicBlock *IncVPBB = Phi->getIncomingBlock(Idx);
+            if (IncVPV != V)
+              continue;
+            if (IncVPBB != VPBB && !VPDT.dominates(VPBB, IncVPBB)) {
----------------
ayalz wrote:

`dominates(VPBB, IncVPBB)` returns true when VPBB == IncVPBB, so suffice to check `if (!VPDT.dominates(VPBB, IncVPBB))`? But is it ok to have VPBB == IncVPBB, given that U is a phi - only if V is a phi too and such dependences between two phi's of the same block are allowed in VPlan, where fixed order dependences are modelled with intervening splices - for VF>1?
In any case good to continue with early continue:
```suggestion
            if (VPDT.properlyDominates(VPBB, IncVPBB))
              continue;
```

https://github.com/llvm/llvm-project/pull/124838