[llvm] [InstCombine] Implement folds of icmp of UCMP/SCMP call and a constant (PR #96118)

[Nikita Popov via llvm-commits]

================
@@ -3926,6 +3926,52 @@ foldICmpUSubSatOrUAddSatWithConstant(ICmpInst::Predicate Pred,
       ConstantInt::get(Op1->getType(), EquivInt));
 }
 
+static Instruction *
+foldICmpOfCmpIntrinsicWithConstant(ICmpInst::Predicate Pred, IntrinsicInst *I,
+                                   const APInt &C,
+                                   InstCombiner::BuilderTy &Builder) {
+  std::optional<ICmpInst::Predicate> NewPredicate = std::nullopt;
+  switch (Pred) {
+  case ICmpInst::ICMP_EQ:
+  case ICmpInst::ICMP_NE:
+    if (C.isZero())
+      NewPredicate = Pred;
+    else if (C.isOne())
+      NewPredicate =
+          Pred == ICmpInst::ICMP_EQ ? ICmpInst::ICMP_UGT : ICmpInst::ICMP_ULE;
+    else if (C.isAllOnes())
+      NewPredicate =
+          Pred == ICmpInst::ICMP_EQ ? ICmpInst::ICMP_ULT : ICmpInst::ICMP_UGE;
+    break;
+
+  case ICmpInst::ICMP_SGT:
+    if (C.isAllOnes())
+      NewPredicate = ICmpInst::ICMP_UGE;
+    else if (C.isZero())
+      NewPredicate = ICmpInst::ICMP_UGT;
+    break;
+
+  case ICmpInst::ICMP_SLT:
+    if (C.isZero())
+      NewPredicate = ICmpInst::ICMP_ULT;
+    else if (C.isOne())
+      NewPredicate = ICmpInst::ICMP_ULE;
+    break;
+
+  default:
+    break;
+  }
+
+  if (!NewPredicate)
+    return nullptr;
+
+  if (I->getIntrinsicID() == Intrinsic::scmp)
+    NewPredicate = ICmpInst::getSignedPredicate(NewPredicate.value());
+  Value *LHS = I->getOperand(0);
+  Value *RHS = I->getOperand(1);
+  return new ICmpInst(NewPredicate.value(), LHS, RHS);
----------------
nikic wrote:

```suggestion
  if (I->getIntrinsicID() == Intrinsic::scmp)
    NewPredicate = ICmpInst::getSignedPredicate(*NewPredicate);
  Value *LHS = I->getOperand(0);
  Value *RHS = I->getOperand(1);
  return new ICmpInst(*NewPredicate, LHS, RHS);
```
Is the preferred style.

https://github.com/llvm/llvm-project/pull/96118