[llvm] [VPlan] Introduce VPScalarPHIRecipe, use for can & EVL IV codegen (NFC). (PR #114305)

[via llvm-commits]

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@@ -1821,3 +1821,24 @@ void VPlanTransforms::createInterleaveGroups(
       }
   }
 }
+
+void VPlanTransforms::prepareToExecute(VPlan &Plan) {
+  ReversePostOrderTraversal<VPBlockDeepTraversalWrapper<VPBlockBase *>> RPOT(
+      Plan.getVectorLoopRegion());
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
+           vp_depth_first_deep(Plan.getEntry()))) {
+    for (VPRecipeBase &R : make_early_inc_range(VPBB->phis())) {
+      if (!isa<VPCanonicalIVPHIRecipe, VPEVLBasedIVPHIRecipe>(&R))
+        continue;
+      auto *PhiR = cast<VPHeaderPHIRecipe>(&R);
+      StringRef Name =
+          isa<VPCanonicalIVPHIRecipe>(PhiR) ? "index" : "evl.based.iv";
+      auto *ScalarR =
+          new VPScalarPHIRecipe(PhiR->getStartValue(), PhiR->getBackedgeValue(),
+                                PhiR->getDebugLoc(), Name);
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ayalz wrote:

Could provide constructors for VPScalarHeaderPHIRecipe from canonical and EVL header phi recipes.

https://github.com/llvm/llvm-project/pull/114305