[llvm] [VPlan] Fix first-order splices without header mask not using EVL (PR #146672)

[Mel Chen via llvm-commits]

================
@@ -2219,16 +2208,37 @@ static void transformRecipestoEVLRecipes(VPlan &Plan, VPValue &EVL) {
                                              DebugLoc());
 
     Builder.setInsertPoint(Header, Header->getFirstNonPhi());
-    PrevEVL = Builder.createScalarPhi({MaxEVL, &EVL}, DebugLoc(), "prev.evl");
+    VPValue *PrevEVL =
+        Builder.createScalarPhi({MaxEVL, &EVL}, DebugLoc(), "prev.evl");
+
+    for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
+             vp_depth_first_deep(Plan.getVectorLoopRegion()->getEntry()))) {
+      for (VPRecipeBase &R : *VPBB) {
+        using namespace VPlanPatternMatch;
+        VPValue *V1, *V2;
+        if (!match(&R,
+                   m_VPInstruction<VPInstruction::FirstOrderRecurrenceSplice>(
+                       m_VPValue(V1), m_VPValue(V2))))
+          continue;
+        VPValue *Imm = Plan.getOrAddLiveIn(
+            ConstantInt::getSigned(Type::getInt32Ty(Ctx), -1));
+        VPWidenIntrinsicRecipe *VPSplice = new VPWidenIntrinsicRecipe(
+            Intrinsic::experimental_vp_splice,
+            {V1, V2, Imm, AllOneMask, PrevEVL, &EVL},
+            TypeInfo.inferScalarType(R.getVPSingleValue()), R.getDebugLoc());
+        VPSplice->insertBefore(&R);
+        R.getVPSingleValue()->replaceAllUsesWith(VPSplice);
+        ToErase.push_back(&R);
+      }
+    }
----------------
Mel-Chen wrote:

Extract 
```
    for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
             vp_depth_first_deep(Plan.getVectorLoopRegion()->getEntry()))) {
```
if possible.

https://github.com/llvm/llvm-project/pull/146672