[llvm] [InstCombine] Fold `(mul (div exact X, C0), C1)` -> `(div exact X, C0/C1)` (PR #96915)

[Yingwei Zheng via llvm-commits]

================
@@ -365,6 +365,28 @@ Instruction *InstCombinerImpl::visitMul(BinaryOperator &I) {
       return BinaryOperator::CreateMul(NegOp0, X);
   }
 
+  if (Op0->hasOneUse()) {
+    // (mul (div exact X, C0), C1)
+    //    -> (div exact C0 / C1)
+    // iff C0 % C1 == 0 and C0 / C1 doesn't create UB.
+    const APInt *C1;
+    auto UDivCheck = [&C1](const APInt &C) { return C.urem(*C1).isZero(); };
+    auto SDivCheck = [&C1](const APInt &C) {
+      APInt Quot, Rem;
+      APInt::sdivrem(C, *C1, Quot, Rem);
+      return Rem.isZero() && !Quot.isMinSignedValue();
----------------
dtcxzyw wrote:

```suggestion
      return Rem.isZero() && !Quot.isAllOnes();
```
See the above comments.



https://github.com/llvm/llvm-project/pull/96915