[llvm] [VPlan] Replace VPRegionBlock with explicit CFG before execute (NFCI). (PR #117506)

[via llvm-commits]

================
@@ -1365,17 +1397,18 @@ void VPlanPrinter::dumpRegion(const VPRegionBlock *Region) {
 
 #endif
 
-/// Returns true if there is a vector loop region and \p VPV is defined in a
-/// loop region.
-static bool isDefinedInsideLoopRegions(const VPValue *VPV) {
-  const VPRecipeBase *DefR = VPV->getDefiningRecipe();
-  return DefR && (!DefR->getParent()->getPlan()->getVectorLoopRegion() ||
-                  DefR->getParent()->getEnclosingLoopRegion());
-}
+bool VPValue::isDefinedOutsideLoop() const {
+  auto *DefR = getDefiningRecipe();
+  if (!DefR)
+    return true;
 
-bool VPValue::isDefinedOutsideLoopRegions() const {
-  return !isDefinedInsideLoopRegions(this);
+  // For non-live-ins, check if is in a region only if the top-level loop region
+  // still exits.
+  const VPBasicBlock *DefVPBB = DefR->getParent();
+  auto *Plan = DefVPBB->getPlan();
+  return Plan->getVectorLoopRegion() && !DefVPBB->getEnclosingLoopRegion();
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ayalz wrote:

De Morgan'izing previous `isDefinedOutsideLoopRegions()`:
```
!(DefR && (!DefR->getParent()->getPlan()->getVectorLoopRegion() ||
           DefR->getParent()->getEnclosingLoopRegion()))
==
!DefR || !(!DefR->getParent()->getPlan()->getVectorLoopRegion() ||
           DefR->getParent()->getEnclosingLoopRegion())
==
!DefR || (DefR->getParent()->getPlan()->getVectorLoopRegion() &&
          !DefR->getParent()->getEnclosingLoopRegion())
```
which appears to be what the current `isDefinedOutsideLoop()` is doing?

This is already conservative in the sense that answering **false** means it **may** be defined inside loops - those that remain after loop regions are removed? Perhaps more accurately called `mustBeDefinedOutsideLoops()`, instead of negating `isDefinedInsideLoopRegions()` which is precisely documented to return "true if there is a vector loop region and \p VPV is defined in a loop region".

https://github.com/llvm/llvm-project/pull/117506