[VPlan] Enforce that there is only ever one header mask. NFC (#152489)

We almost only ever have one header mask, except with the data tail
folding style, i.e. with VPInstruction::ActiveLaneMask.

All we need to do is to make sure to erase the old header icmp based
header mask when replacing it.

Author: lukel97

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -2051,11 +2051,11 @@ static VPActiveLaneMaskPHIRecipe *addVPLaneMaskPhiAndUpdateExitBranch(
   return LaneMaskPhi;
 }
 
-/// Collect all VPValues representing a header mask through the (ICMP_ULE,
-/// WideCanonicalIV, backedge-taken-count) pattern.
+/// Collect the header mask with the pattern:
+///   (ICMP_ULE, WideCanonicalIV, backedge-taken-count)
 /// TODO: Introduce explicit recipe for header-mask instead of searching
 /// for the header-mask pattern manually.
-static SmallVector<VPValue *> collectAllHeaderMasks(VPlan &Plan) {
+static VPSingleDefRecipe *findHeaderMask(VPlan &Plan) {
   SmallVector<VPValue *> WideCanonicalIVs;
   auto *FoundWidenCanonicalIVUser =
       find_if(Plan.getCanonicalIV()->users(),
@@ -2079,21 +2079,22 @@ static SmallVector<VPValue *> collectAllHeaderMasks(VPlan &Plan) {
       WideCanonicalIVs.push_back(WidenOriginalIV);
   }
 
-  // Walk users of wide canonical IVs and collect to all compares of the form
+  // Walk users of wide canonical IVs and find the single compare of the form
   // (ICMP_ULE, WideCanonicalIV, backedge-taken-count).
-  SmallVector<VPValue *> HeaderMasks;
+  VPSingleDefRecipe *HeaderMask = nullptr;
   for (auto *Wide : WideCanonicalIVs) {
     for (VPUser *U : SmallVector<VPUser *>(Wide->users())) {
-      auto *HeaderMask = dyn_cast<VPInstruction>(U);
-      if (!HeaderMask || !vputils::isHeaderMask(HeaderMask, Plan))
+      auto *VPI = dyn_cast<VPInstruction>(U);
+      if (!VPI || !vputils::isHeaderMask(VPI, Plan))
         continue;
 
-      assert(HeaderMask->getOperand(0) == Wide &&
+      assert(VPI->getOperand(0) == Wide &&
              "WidenCanonicalIV must be the first operand of the compare");
-      HeaderMasks.push_back(HeaderMask);
+      assert(!HeaderMask && "Multiple header masks found?");
+      HeaderMask = VPI;
     }
   }
-  return HeaderMasks;
+  return HeaderMask;
 }
 
 void VPlanTransforms::addActiveLaneMask(
@@ -2109,6 +2110,7 @@ void VPlanTransforms::addActiveLaneMask(
               [](VPUser *U) { return isa<VPWidenCanonicalIVRecipe>(U); });
   assert(FoundWidenCanonicalIVUser &&
          "Must have widened canonical IV when tail folding!");
+  VPSingleDefRecipe *HeaderMask = findHeaderMask(Plan);
   auto *WideCanonicalIV =
       cast<VPWidenCanonicalIVRecipe>(*FoundWidenCanonicalIVUser);
   VPSingleDefRecipe *LaneMask;
@@ -2122,11 +2124,11 @@ void VPlanTransforms::addActiveLaneMask(
                               "active.lane.mask");
   }
 
-  // Walk users of WideCanonicalIV and replace all compares of the form
-  // (ICMP_ULE, WideCanonicalIV, backedge-taken-count) with an
-  // active-lane-mask.
-  for (VPValue *HeaderMask : collectAllHeaderMasks(Plan))
-    HeaderMask->replaceAllUsesWith(LaneMask);
+  // Walk users of WideCanonicalIV and replace the header mask of the form
+  // (ICMP_ULE, WideCanonicalIV, backedge-taken-count) with an active-lane-mask,
+  // removing the old one to ensure there is always only a single header mask.
+  HeaderMask->replaceAllUsesWith(LaneMask);
+  HeaderMask->eraseFromParent();
 }
 
 /// Try to optimize a \p CurRecipe masked by \p HeaderMask to a corresponding
@@ -2252,6 +2254,10 @@ static void transformRecipestoEVLRecipes(VPlan &Plan, VPValue &EVL) {
     }
   }
 
+  VPValue *HeaderMask = findHeaderMask(Plan);
+  if (!HeaderMask)
+    return;
+
   // Replace header masks with a mask equivalent to predicating by EVL:
   //
   // icmp ule widen-canonical-iv backedge-taken-count
@@ -2263,10 +2269,8 @@ static void transformRecipestoEVLRecipes(VPlan &Plan, VPValue &EVL) {
   VPValue *EVLMask = Builder.createICmp(
       CmpInst::ICMP_ULT,
       Builder.createNaryOp(VPInstruction::StepVector, {}, EVLType), &EVL);
-  for (VPValue *HeaderMask : collectAllHeaderMasks(Plan)) {
-    HeaderMask->replaceAllUsesWith(EVLMask);
-    ToErase.push_back(HeaderMask->getDefiningRecipe());
-  }
+  HeaderMask->replaceAllUsesWith(EVLMask);
+  ToErase.push_back(HeaderMask->getDefiningRecipe());
 
   // Try to optimize header mask recipes away to their EVL variants.
   // TODO: Split optimizeMaskToEVL out and move into