[llvm] [ConstraintElim] Simplify `sadd_with_overflow` if A and B have different signs (PR #135784)

[Yingwei Zheng via llvm-commits]

================
@@ -1719,17 +1752,39 @@ tryToSimplifyOverflowMath(IntrinsicInst *II, ConstraintInfo &Info,
   };
 
   bool Changed = false;
-  if (II->getIntrinsicID() == Intrinsic::ssub_with_overflow) {
+  Value *A = II->getArgOperand(0);
+  Value *B = II->getArgOperand(1);
+  Type *Ty = A->getType();
+  auto *Zero = ConstantInt::get(Ty, 0);
+
+  switch (II->getIntrinsicID()) {
+  default:
+    llvm_unreachable("Unexpected intrinsic.");
+  case Intrinsic::sadd_with_overflow: {
+    bool ASgeZero = DoesConditionHold(CmpInst::ICMP_SGE, A, Zero, Info);
+    bool BSgeZero = DoesConditionHold(CmpInst::ICMP_SGE, B, Zero, Info);
+    bool ASleZero = DoesConditionHold(CmpInst::ICMP_SLE, A, Zero, Info);
+    bool BSleZero = DoesConditionHold(CmpInst::ICMP_SLE, B, Zero, Info);
+
+    // If A and B have different signs, sadd.with.overflow(a, b) should not
+    // overflow.
+    if ((ASgeZero && BSleZero) || (ASleZero && BSgeZero)) {
----------------
dtcxzyw wrote:

```suggestion
    if ((DoesConditionHold(CmpInst::ICMP_SGE, A, Zero, Info) && DoesConditionHold(CmpInst::ICMP_SLE, B, Zero, Info)) || (DoesConditionHold(CmpInst::ICMP_SLE, A, Zero, Info) && DoesConditionHold(CmpInst::ICMP_SGE, B, Zero, Info))) {
```
`DoesConditionHold` is expensive. 

https://github.com/llvm/llvm-project/pull/135784