[VPlan] Remove CanonicalIV when dissolving loop regions (NFC). (#142372)

Directly replace the canonical IV when we dissolve the containing
region. That ensures that it won't get removed before the region gets
removed, which would result in an invalid region.

This removes the current ordering constraint between
convertToConcreteRecipes and dissolving regions.

PR: https://github.com/llvm/llvm-project/pull/142372

llvm/lib/Transforms/Vectorize/VPlan.cpp

diff --git a/llvm/lib/Transforms/Vectorize/VPlan.cpp b/llvm/lib/Transforms/Vectorize/VPlan.cpp
index 165b57c..280ea47 100644
--- a/llvm/lib/Transforms/Vectorize/VPlan.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlan.cpp
@@ -883,6 +883,16 @@
 
 void VPRegionBlock::dissolveToCFGLoop() {
   auto *Header = cast<VPBasicBlock>(getEntry());
+  if (auto *CanIV = dyn_cast<VPCanonicalIVPHIRecipe>(&Header->front())) {
+    assert(this == getPlan()->getVectorLoopRegion() &&
+           "Canonical IV must be in the entry of the top-level loop region");
+    auto *ScalarR = VPBuilder(CanIV).createScalarPhi(
+        {CanIV->getStartValue(), CanIV->getBackedgeValue()},
+        CanIV->getDebugLoc(), "index");
+    CanIV->replaceAllUsesWith(ScalarR);
+    CanIV->eraseFromParent();
+  }
+
   VPBlockBase *Preheader = getSinglePredecessor();
   auto *ExitingLatch = cast<VPBasicBlock>(getExiting());
   VPBlockBase *Middle = getSingleSuccessor();

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

diff --git a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
index 3481001..ea617f0 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
@@ -2599,14 +2599,10 @@
   for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
            vp_depth_first_deep(Plan.getEntry()))) {
     for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
-      if (isa<VPCanonicalIVPHIRecipe, VPEVLBasedIVPHIRecipe>(&R)) {
-        auto *PhiR = cast<VPHeaderPHIRecipe>(&R);
-        StringRef Name =
-            isa<VPCanonicalIVPHIRecipe>(PhiR) ? "index" : "evl.based.iv";
-        VPBuilder Builder(PhiR);
-        auto *ScalarR = Builder.createScalarPhi(
+      if (auto *PhiR = dyn_cast<VPEVLBasedIVPHIRecipe>(&R)) {
+        auto *ScalarR = VPBuilder(PhiR).createScalarPhi(
             {PhiR->getStartValue(), PhiR->getBackedgeValue()},
-            PhiR->getDebugLoc(), Name);
+            PhiR->getDebugLoc(), "evl.based.iv");
         PhiR->replaceAllUsesWith(ScalarR);
         ToRemove.push_back(PhiR);
         continue;