[llvm] [llvm][instcombine] Add Missed Optimization for Folding Min Max intrinsic into PHI instruction (PR #84619)

[via llvm-commits]

================
@@ -611,6 +611,95 @@ Instruction *InstCombinerImpl::foldPHIArgGEPIntoPHI(PHINode &PN) {
   return NewGEP;
 }
 
+/// helper function for foldPHIWithMinMax
+Instruction *
+InstCombinerImpl::foldPHIWithMinMaxHelper(PHINode &PN, Instruction *I, Value *Z,
+                                          ICmpInst::Predicate Pred) {
+
+  auto IsCondKnownTrue = [](Value *Val) -> std::optional<bool> {
+    if (!Val)
+      return std::nullopt;
+    if (match(Val, m_One()))
+      return true;
+    if (match(Val, m_Zero()))
+      return false;
+    return std::nullopt;
+  };
+
+  ICmpInst::Predicate SwappedPred =
+      ICmpInst::getNonStrictPredicate(ICmpInst::getSwappedPredicate(Pred));
+  for (unsigned OpNum = 0; OpNum != PN.getNumIncomingValues(); ++OpNum) {
+    if (auto *MinMax = dyn_cast<MinMaxIntrinsic>(PN.getIncomingValue(OpNum))) {
+      if (Pred != MinMax->getPredicate())
+        continue;
+
+      Value *X = MinMax->getLHS();
+      Value *Y = MinMax->getRHS();
+
+      SimplifyQuery Q = SQ.getWithInstruction(I);
+
+      auto CmpXZ = IsCondKnownTrue(simplifyICmpInst(SwappedPred, X, Z, Q));
+      auto CmpYZ = IsCondKnownTrue(simplifyICmpInst(SwappedPred, Y, Z, Q));
+
+      if (!CmpXZ.has_value() && !CmpYZ.has_value())
+        continue;
+      if (CmpXZ.has_value() && CmpYZ.has_value())
----------------
PeterChou1 wrote:

You would take the smaller of two if it was a min intrinsic and the bigger of the two if it was max intrinsic. In retrospect when I made the pull request I thought this case was more complicated than I first thought so I skipped implementing it. 
I'll amend this when I refactor the code to put it to put it in InstCombineCompares as noted by @nikic 

https://github.com/llvm/llvm-project/pull/84619