[SLP]Fix PR79321: SLPVectorizer's PHICompare doesn't provide a strict

weak ordering.
Try to make PHICompare to meat strict weak ordering criteria.

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -15953,6 +15953,8 @@ bool SLPVectorizerPass::vectorizeChainsInBlock(BasicBlock *BB, BoUpSLP &R) {
     for (int I = 0, E = Opcodes1.size(); I < E; ++I) {
       // Undefs are compatible with any other value.
       if (isa<UndefValue>(Opcodes1[I]) || isa<UndefValue>(Opcodes2[I])) {
+        if (isa<UndefValue>(Opcodes1[I]) && isa<UndefValue>(Opcodes2[I]))
+          continue;
         if (isa<Instruction>(Opcodes1[I]))
           return true;
         if (isa<Instruction>(Opcodes2[I]))
@@ -15961,9 +15963,11 @@ bool SLPVectorizerPass::vectorizeChainsInBlock(BasicBlock *BB, BoUpSLP &R) {
           return true;
         if (isa<Constant>(Opcodes2[I]) && !isa<UndefValue>(Opcodes2[I]))
           return false;
-        if (isa<UndefValue>(Opcodes1[I]) && isa<UndefValue>(Opcodes2[I]))
-          continue;
-        return isa<UndefValue>(Opcodes2[I]);
+        if (isa<UndefValue>(Opcodes1[I]) && !isa<UndefValue>(Opcodes2[I]))
+          return false;
+        if (!isa<UndefValue>(Opcodes1[I]) && isa<UndefValue>(Opcodes2[I]))
+          return true;
+        continue;
       }
       if (auto *I1 = dyn_cast<Instruction>(Opcodes1[I]))
         if (auto *I2 = dyn_cast<Instruction>(Opcodes2[I])) {
@@ -15984,14 +15988,14 @@ bool SLPVectorizerPass::vectorizeChainsInBlock(BasicBlock *BB, BoUpSLP &R) {
           return I1->getOpcode() < I2->getOpcode();
         }
       if (isa<Constant>(Opcodes1[I]) && isa<Constant>(Opcodes2[I]))
-        return Opcodes1[I]->getValueID() < Opcodes2[I]->getValueID();
-      if (isa<Instruction>(Opcodes1[I]))
+        continue;
+      if (isa<Instruction>(Opcodes1[I]) && !isa<Instruction>(Opcodes2[I]))
         return true;
-      if (isa<Instruction>(Opcodes2[I]))
+      if (!isa<Instruction>(Opcodes1[I]) && isa<Instruction>(Opcodes2[I]))
         return false;
-      if (isa<Constant>(Opcodes1[I]))
+      if (isa<Constant>(Opcodes1[I]) && !isa<Constant>(Opcodes2[I]))
         return true;
-      if (isa<Constant>(Opcodes2[I]))
+      if (!isa<Constant>(Opcodes1[I]) && isa<Constant>(Opcodes2[I]))
         return false;
       if (Opcodes1[I]->getValueID() < Opcodes2[I]->getValueID())
         return true;