[VPlan] Verify plan before optimizations. NFC (#122678)

I've been exploring verifying the VPlan before and after the EVL
transformation steps, and noticed that the VPlan comes out in an invalid
state between construction and optimisation.

In adjustRecipesForReductions, we leave behind some dead recipes which
are invalid:

1) When we replace a link with a reduction recipe, the old link ends up
becoming a use-before-def:

    WIDEN ir<%l7> = add ir<%sum.02>, ir<%indvars.iv>.1
    WIDEN ir<%l8> = add ir<%l7>.1, ir<%l3>
    WIDEN ir<%l9> = add ir<%l8>.1, ir<%l5>
    ...
    REDUCE ir<%l7>.1 = ir<%sum.02> + reduce.add (ir<%indvars.iv>.1)
    REDUCE ir<%l8>.1 = ir<%l7>.1 + reduce.add (ir<%l3>)
    REDUCE ir<%l9>.1 = ir<%l8>.1 + reduce.add (ir<%l5>)

2) When transforming an AnyOf reduction phi to a boolean, we leave
behind a select with mismatching operand types, which will trigger the
assertions in VTypeAnalysis after #122679

This adds an extra verification step and deletes the dead recipes
eagerly to keep the plan valid.

Author: lukel97

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -9624,6 +9624,8 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
     VPlanTransforms::addActiveLaneMask(*Plan, ForControlFlow,
                                        WithoutRuntimeCheck);
   }
+
+  assert(verifyVPlanIsValid(*Plan) && "VPlan is invalid");
   return Plan;
 }
 
@@ -9698,6 +9700,8 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
   VPRegionBlock *VectorLoopRegion = Plan->getVectorLoopRegion();
   VPBasicBlock *Header = VectorLoopRegion->getEntryBasicBlock();
   VPBasicBlock *MiddleVPBB = Plan->getMiddleBlock();
+  SmallVector<VPRecipeBase *> ToDelete;
+
   for (VPRecipeBase &R : Header->phis()) {
     auto *PhiR = dyn_cast<VPReductionPHIRecipe>(&R);
     if (!PhiR || !PhiR->isInLoop() || (MinVF.isScalar() && !PhiR->isOrdered()))
@@ -9817,10 +9821,11 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
           CM.useOrderedReductions(RdxDesc), CurrentLinkI->getDebugLoc());
       // Append the recipe to the end of the VPBasicBlock because we need to
       // ensure that it comes after all of it's inputs, including CondOp.
-      // Note that this transformation may leave over dead recipes (including
-      // CurrentLink), which will be cleaned by a later VPlan transform.
+      // Delete CurrentLink as it will be invalid if its operand is replaced
+      // with a reduction defined at the bottom of the block in the next link.
       LinkVPBB->appendRecipe(RedRecipe);
       CurrentLink->replaceAllUsesWith(RedRecipe);
+      ToDelete.push_back(CurrentLink);
       PreviousLink = RedRecipe;
     }
   }
@@ -9935,6 +9940,8 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
         Cmp = Builder.createNot(Cmp);
       VPValue *Or = Builder.createOr(PhiR, Cmp);
       Select->getVPSingleValue()->replaceAllUsesWith(Or);
+      // Delete Select now that it has invalid types.
+      ToDelete.push_back(Select);
 
       // Convert the reduction phi to operate on bools.
       PhiR->setOperand(0, Plan->getOrAddLiveIn(ConstantInt::getFalse(
@@ -9952,6 +9959,8 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
   }
 
   VPlanTransforms::clearReductionWrapFlags(*Plan);
+  for (VPRecipeBase *R : ToDelete)
+    R->eraseFromParent();
 }
 
 void VPDerivedIVRecipe::execute(VPTransformState &State) {