[VPlan] Check users of LoopExitInstDef in VPlan directly. (NFCI)

Instead of walking the IR def use chains of the generated code, adjust
the generated VPInstruction if needed and check its users in VPlan.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -3818,34 +3818,16 @@ void InnerLoopVectorizer::fixReduction(VPReductionPHIRecipe *PhiR,
   // instead of the former. For an inloop reduction the reduction will already
   // be predicated, and does not need to be handled here.
   if (Cost->foldTailByMasking() && !PhiR->isInLoop()) {
-    for (unsigned Part = 0; Part < UF; ++Part) {
-      Value *VecLoopExitInst = State.get(LoopExitInstDef, Part);
-      SelectInst *Sel = nullptr;
-      for (User *U : VecLoopExitInst->users()) {
-        if (isa<SelectInst>(U)) {
-          assert((!Sel || U == Sel) &&
-                 "Reduction exit feeding two different selects");
-          Sel = cast<SelectInst>(U);
-        } else
-          assert(isa<PHINode>(U) && "Reduction exit must feed Phi's or select");
-      }
-      assert(Sel && "Reduction exit feeds no select");
-      State.reset(LoopExitInstDef, Sel, Part);
-
-      // If the target can create a predicated operator for the reduction at no
-      // extra cost in the loop (for example a predicated vadd), it can be
-      // cheaper for the select to remain in the loop than be sunk out of it,
-      // and so use the select value for the phi instead of the old
-      // LoopExitValue.
-      if (PreferPredicatedReductionSelect ||
-          TTI->preferPredicatedReductionSelect(
-              RdxDesc.getOpcode(), PhiTy,
-              TargetTransformInfo::ReductionFlags())) {
-        auto *VecRdxPhi =
-            cast<PHINode>(State.get(PhiR, Part));
-        VecRdxPhi->setIncomingValueForBlock(VectorLoopLatch, Sel);
+    VPValue *Def = nullptr;
+    for (VPUser *U : LoopExitInstDef->users()) {
+      auto *S = dyn_cast<VPInstruction>(U);
+      if (S && S->getOpcode() == Instruction::Select) {
+        Def = S;
+        break;
       }
     }
+    if (Def)
+      LoopExitInstDef = Def;
   }
 
   // If the vector reduction can be performed in a smaller type, we truncate
@@ -9099,6 +9081,11 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
               ? new VPInstruction(Instruction::Select, {Cond, Red, PhiR}, FMFs)
               : new VPInstruction(Instruction::Select, {Cond, Red, PhiR});
       Select->insertBefore(&*Builder.getInsertPoint());
+      if (PreferPredicatedReductionSelect ||
+          TTI.preferPredicatedReductionSelect(
+              PhiR->getRecurrenceDescriptor().getOpcode(), PhiTy,
+              TargetTransformInfo::ReductionFlags()))
+        PhiR->setOperand(1, Select);
     }
   }