[llvm] [NFC] SLVectorizer comparator refactoring that preserves behavior (PR #84966)

[David Blaikie via llvm-commits]

================
@@ -16661,20 +16636,44 @@ bool SLPVectorizerPass::vectorizeChainsInBlock(BasicBlock *BB, BoUpSLP &R) {
             continue;
           return I1->getOpcode() < I2->getOpcode();
         }
-      if (isa<Constant>(Opcodes1[I]) && isa<Constant>(Opcodes2[I]))
-        continue;
-      if (isa<Instruction>(Opcodes1[I]) && !isa<Instruction>(Opcodes2[I]))
-        return true;
-      if (!isa<Instruction>(Opcodes1[I]) && isa<Instruction>(Opcodes2[I]))
-        return false;
-      if (isa<Constant>(Opcodes1[I]) && !isa<Constant>(Opcodes2[I]))
-        return true;
-      if (!isa<Constant>(Opcodes1[I]) && isa<Constant>(Opcodes2[I]))
-        return false;
-      if (Opcodes1[I]->getValueID() < Opcodes2[I]->getValueID())
-        return true;
-      if (Opcodes1[I]->getValueID() > Opcodes2[I]->getValueID())
-        return false;
+        if (I1)
+          return true;
+        if (I2)
+          return false;
+      }
+      {
+        // Non-undef constants come next.
+        bool C1 = isa<Constant>(Opcodes1[I]) && !isa<UndefValue>(Opcodes1[I]);
+        bool C2 = isa<Constant>(Opcodes2[I]) && !isa<UndefValue>(Opcodes2[I]);
+        if (C1 && C2)
+          continue;
+        if (C1)
+          return true;
+        if (C2)
+          return false;
+      }
+      bool U1 = isa<UndefValue>(Opcodes1[I]);
+      bool U2 = isa<UndefValue>(Opcodes2[I]);
+      {
+        // Non-constant non-instructions come next.
+        if (!U1 && !U2) {
+          auto ValID1 = Opcodes1[I]->getValueID();
+          auto ValID2 = Opcodes2[I]->getValueID();
+          if (ValID1 == ValID2)
+            continue;
+          if (ValID1 < ValID2)
+            return true;
+          if (ValID1 > ValID2)
+            return false;
+        }
+        if (!U1)
+          return true;
+        if (!U2)
+          return false;
+      }
+      // Undefs come last.
+      assert(U1 && U2);
----------------
dwblaikie wrote:

Ah, sure - added in 9ac03158987802706110ef465c4b6a7553cc4b86

https://github.com/llvm/llvm-project/pull/84966