[llvm] [VPlan] Introduce recipes for VP loads and stores. (PR #87816)

[via llvm-commits]

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@@ -1336,6 +1340,31 @@ void VPlanTransforms::addExplicitVectorLength(VPlan &Plan) {
   NextEVLIV->insertBefore(CanonicalIVIncrement);
   EVLPhi->addOperand(NextEVLIV);
 
+  for (VPValue *HeaderMask : collectAllHeaderMasks(Plan)) {
+    for (VPUser *U : collectUsersRecursively(HeaderMask)) {
+      auto *MemR = dyn_cast<VPWidenMemoryRecipe>(U);
+      if (!MemR)
+        continue;
+      assert(!MemR->isReverse() &&
+             "Reversed memory operations not supported yet.");
+      VPValue *OrigMask = MemR->getMask();
+      assert(OrigMask && "Unmasked widen memory recipe when folding tail");
+      VPValue *NewMask = HeaderMask == OrigMask ? nullptr : OrigMask;
+      if (auto *L = dyn_cast<VPWidenLoadRecipe>(MemR)) {
+        auto *N = new VPWidenEVLLoadRecipe(L, VPEVL, NewMask);
+        N->insertBefore(L);
+        L->replaceAllUsesWith(N);
+        L->eraseFromParent();
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ayalz wrote:

```suggestion
```
nit: could the following `recursivelyDeleteDeadRecipes()` take care of erasing Load recipes from their parents? The former is needed due to no subsequent VPlan dce.
Collecting dead Stores is more challenging.

https://github.com/llvm/llvm-project/pull/87816