[VPlan] Don't emit VPBlendRecipes with only one incoming value. NFC (#171804)

We can just directly use the incoming value. These single value blends
would get optimized later on in simplifyBlends, but by doing it early it
removes the notion of an "immediately normalized" blend, and simplifies
an upcoming patch.

Author: lukel97

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -2517,7 +2517,7 @@ class LLVM_ABI_FOR_TEST VPBlendRecipe : public VPSingleDefRecipe {
   /// all other incoming values are merged into it.
   VPBlendRecipe(PHINode *Phi, ArrayRef<VPValue *> Operands, DebugLoc DL)
       : VPSingleDefRecipe(VPDef::VPBlendSC, Operands, Phi, DL) {
-    assert(Operands.size() > 0 && "Expected at least one operand!");
+    assert(Operands.size() >= 2 && "Expected at least two operands!");
   }
 
   VPBlendRecipe *clone() override {

llvm/lib/Transforms/Vectorize/VPlanPredicator.cpp

@@ -240,17 +240,16 @@ void VPPredicator::convertPhisToBlends(VPBasicBlock *VPBB) {
     // be duplications since this is a simple recursive scan, but future
     // optimizations will clean it up.
 
+    if (all_equal(PhiR->incoming_values())) {
+      PhiR->replaceAllUsesWith(PhiR->getIncomingValue(0));
+      PhiR->eraseFromParent();
+      continue;
+    }
+
     SmallVector<VPValue *, 2> OperandsWithMask;
     for (const auto &[InVPV, InVPBB] : PhiR->incoming_values_and_blocks()) {
       OperandsWithMask.push_back(InVPV);
-      VPValue *EdgeMask = getEdgeMask(InVPBB, VPBB);
-      if (!EdgeMask) {
-        assert(all_equal(PhiR->incoming_values()) &&
-               "Distinct incoming values with one having a full mask");
-        break;
-      }
-
-      OperandsWithMask.push_back(EdgeMask);
+      OperandsWithMask.push_back(getEdgeMask(InVPBB, VPBB));
     }
     PHINode *IRPhi = cast_or_null<PHINode>(PhiR->getUnderlyingValue());
     auto *Blend =

llvm/unittests/Transforms/Vectorize/VPlanVerifierTest.cpp

@@ -103,7 +103,7 @@ TEST_F(VPVerifierTest, VPBlendUseBeforeDefDifferentBB) {
   auto *CanIV = new VPCanonicalIVPHIRecipe(Zero, {});
   VPInstruction *BranchOnCond =
       new VPInstruction(VPInstruction::BranchOnCond, {CanIV});
-  auto *Blend = new VPBlendRecipe(Phi, {DefI}, {});
+  auto *Blend = new VPBlendRecipe(Phi, {DefI, Plan.getTrue()}, {});
 
   VPBasicBlock *VPBB1 = Plan.getEntry();
   VPBasicBlock *VPBB2 = Plan.createVPBasicBlock("");