[llvm] [LV][EVL] Support reduction idioms using tail folding with EVL. (PR #90184)

[Mel Chen via llvm-commits]

================
@@ -1358,23 +1358,30 @@ bool VPlanTransforms::tryAddExplicitVectorLength(VPlan &Plan) {
 
   for (VPValue *HeaderMask : collectAllHeaderMasks(Plan)) {
     for (VPUser *U : collectUsersRecursively(HeaderMask)) {
-      auto *MemR = dyn_cast<VPWidenMemoryRecipe>(U);
-      if (!MemR)
-        continue;
-      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 VPWidenLoadEVLRecipe(L, VPEVL, NewMask);
-        N->insertBefore(L);
-        L->replaceAllUsesWith(N);
-        L->eraseFromParent();
-      } else if (auto *S = dyn_cast<VPWidenStoreRecipe>(MemR)) {
-        auto *N = new VPWidenStoreEVLRecipe(S, VPEVL, NewMask);
-        N->insertBefore(S);
-        S->eraseFromParent();
-      } else {
-        llvm_unreachable("unsupported recipe");
+      auto GetNewMask = [&](VPValue *OrigMask) -> VPValue * {
+        assert(OrigMask && "Unmasked recipe when folding tail");
+        return HeaderMask == OrigMask ? nullptr : OrigMask;
+      };
+      if (auto *MemR = dyn_cast<VPWidenMemoryRecipe>(U)) {
+        VPValue *NewMask = GetNewMask(MemR->getMask());
+        if (auto *L = dyn_cast<VPWidenLoadRecipe>(MemR)) {
+          auto *N = new VPWidenLoadEVLRecipe(L, VPEVL, NewMask);
+          N->insertBefore(L);
----------------
Mel-Chen wrote:

05dd9eeae0ebcd5a5da65f7a58b07327f7530672
I plan to extract the recipe replacement into namespace vputils. Maybe like `ReplaceInstWithInst` in BasicBlockUtils.h.

https://github.com/llvm/llvm-project/pull/90184