[llvm] [VPlan] Make canonical IV part of the region (PR #156262)

[via llvm-commits]

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@@ -846,18 +858,30 @@ void VPRegionBlock::print(raw_ostream &O, const Twine &Indent,
 
 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");
+  auto *ExitingLatch = cast<VPBasicBlock>(getExiting());
+  VPValue *CanIV = getCanonicalIV();
+  if (CanIV && CanIV->getNumUsers() > 0) {
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ayalz wrote:

Worth documenting somewhere that CanIV should continue to exist (until loop region dissolves) even w/o users, i.e., avoid dce, as its existence indicates region loopness, and provides other information such as type; plus users might conceptually be introduced late(?)

https://github.com/llvm/llvm-project/pull/156262