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

Spinning off from #79321 / 35f4592 - looked like the comparator could be
simplified & made more clear/less risk of leaving hidden bugs.

Author: dwblaikie

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -16614,36 +16614,11 @@ bool SLPVectorizerPass::vectorizeChainsInBlock(BasicBlock *BB, BoUpSLP &R) {
     if (Opcodes1.size() > Opcodes2.size())
       return false;
     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])) {
-          assert(isa<UndefValue>(Opcodes2[I]) && "Expected 2nd undef value");
-          return true;
-        }
-        if (isa<Instruction>(Opcodes2[I])) {
-          assert(isa<UndefValue>(Opcodes1[I]) && "Expected 1st undef value");
-          return false;
-        }
-        if (isa<Constant>(Opcodes1[I]) && !isa<UndefValue>(Opcodes1[I])) {
-          assert(isa<UndefValue>(Opcodes2[I]) && "Expected 2nd undef value");
-          return true;
-        }
-        if (isa<Constant>(Opcodes2[I]) && !isa<UndefValue>(Opcodes2[I])) {
-          assert(isa<UndefValue>(Opcodes1[I]) && "Expected 1st undef value");
-          return false;
-        }
-        if (!isa<UndefValue>(Opcodes2[I])) {
-          assert(isa<UndefValue>(Opcodes1[I]) && "Expected 1st undef value");
-          return false;
-        }
-        assert(!isa<UndefValue>(Opcodes1[I]) && isa<UndefValue>(Opcodes2[I]) &&
-               "Expected 1st non-undef and 2nd undef value");
-        return true;
-      }
-      if (auto *I1 = dyn_cast<Instruction>(Opcodes1[I]))
-        if (auto *I2 = dyn_cast<Instruction>(Opcodes2[I])) {
+      {
+        // Instructions come first.
+        auto *I1 = dyn_cast<Instruction>(Opcodes1[I]);
+        auto *I2 = dyn_cast<Instruction>(Opcodes2[I]);
+        if (I1 && I2) {
           DomTreeNodeBase<BasicBlock> *NodeI1 = DT->getNode(I1->getParent());
           DomTreeNodeBase<BasicBlock> *NodeI2 = DT->getNode(I2->getParent());
           if (!NodeI1)
@@ -16660,20 +16635,44 @@ 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);
+      continue;
     }
     return false;
   };