[llvm] [InstSimplify] Implement simple folds for `ucmp`/`scmp` intrinsics (PR #95601)

[Yingwei Zheng via llvm-commits]

================
@@ -6505,6 +6505,29 @@ Value *llvm::simplifyBinaryIntrinsic(Intrinsic::ID IID, Type *ReturnType,
 
     break;
   }
+  case Intrinsic::scmp:
+  case Intrinsic::ucmp: {
+    // Fold cmp x, x -> 0
+    if (Op0 == Op1)
+      return Constant::getNullValue(ReturnType);
+
+    // Fold to a constant if the relationship between operands can be
+    // established with certainty
+    if (isICmpTrue(CmpInst::ICMP_EQ, Op0, Op1, Q, RecursionLimit))
+      return Constant::getNullValue(ReturnType);
+
+    ICmpInst::Predicate PredGT =
+        IID == Intrinsic::scmp ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT;
+    if (isICmpTrue(PredGT, Op0, Op1, Q, RecursionLimit))
+      return ConstantInt::get(ReturnType, 1);
+
+    ICmpInst::Predicate PredLT =
+        IID == Intrinsic::scmp ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT;
+    if (isICmpTrue(PredLT, Op0, Op1, Q, RecursionLimit))
+      return ConstantInt::get(ReturnType, -1, /*IsSigned*/ true);
----------------
dtcxzyw wrote:

```suggestion
      return ConstantInt::getSigned(ReturnType, -1);
```

https://github.com/llvm/llvm-project/pull/95601