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

[via llvm-commits]

================
@@ -1070,10 +1070,9 @@ void VPlan::execute(VPTransformState *State) {
     }
 
     auto *PhiR = cast<VPHeaderPHIRecipe>(&R);
-    bool NeedsScalar =
-        isa<VPCanonicalIVPHIRecipe, VPEVLBasedIVPHIRecipe>(PhiR) ||
-        (isa<VPReductionPHIRecipe>(PhiR) &&
-         cast<VPReductionPHIRecipe>(PhiR)->isInLoop());
+    bool NeedsScalar = isa<VPScalarPHIRecipe>(PhiR) ||
+                       (isa<VPReductionPHIRecipe>(PhiR) &&
+                        cast<VPReductionPHIRecipe>(PhiR)->isInLoop());
----------------
ayalz wrote:

In-loop reductions should also be converted to VPScalarHeaderPHIRecipe?

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