[llvm] [VPlan] Implement interleaving as VPlan-to-VPlan transform. (PR #95842)

[via llvm-commits]

================
@@ -3665,6 +3690,29 @@ inline bool isUniformAfterVectorization(VPValue *VPV) {
     return VPI->isVectorToScalar();
   return false;
 }
+
+/// Checks if \p C is uniform across all VFs and UFs. It is considered as such
+/// if it is either defined outside the vector region or its operand is known to
+/// be uniform across all VFs and UFs (e.g. VPDerivedIV or VPCanonicalIVPHI).
+inline bool isUniformAcrossVFsAndUFs(VPValue *V) {
+  if (auto *VPI = dyn_cast_or_null<VPInstruction>(V->getDefiningRecipe())) {
+    return VPI ==
+           VPI->getParent()->getPlan()->getCanonicalIV()->getBackedgeValue();
+  }
+  if (isa<VPCanonicalIVPHIRecipe, VPDerivedIVRecipe, VPExpandSCEVRecipe>(V))
+    return true;
+  if (isa<VPReplicateRecipe>(V) && cast<VPReplicateRecipe>(V)->isUniform() &&
+      (isa<LoadInst, StoreInst>(V->getUnderlyingValue())) &&
+      all_of(V->getDefiningRecipe()->operands(),
+             [](VPValue *Op) { return Op->isDefinedOutsideVectorRegions(); }))
+    return true;
----------------
ayalz wrote:

```suggestion
  // Loads and stores that are uniform across VF lanes are handled by VPReplicateRecipe.IsUniform. They are also uniform across UF parts if all their operands are invariant:
  if (isa<VPReplicateRecipe>(V) && cast<VPReplicateRecipe>(V)->isUniform() &&
      (isa<LoadInst, StoreInst>(V->getUnderlyingValue())) &&
      all_of(R->operands(),
             [](VPValue *Op) { return Op->isDefinedOutsideVectorRegions(); }))
    return true;
```

https://github.com/llvm/llvm-project/pull/95842