[llvm] [InstCombine] Fold (X << Y) / (X << Z) -> 1 << Y >> Z (PR #68863)

[Yingwei Zheng via llvm-commits]

================
@@ -980,6 +980,26 @@ static Instruction *foldIDivShl(BinaryOperator &I,
       Ret = BinaryOperator::CreateSDiv(X, Y);
   }
 
+  // If X << Y and X << Z does not overflow, then:
+  // (X << Y) / (X << Z) -> (1 << Y) / (1 << Z) -> 1 << Y >> Z
+  if (match(Op0, m_Shl(m_Value(X), m_Value(Y))) &&
+      match(Op1, m_Shl(m_Specific(X), m_Value(Z)))) {
+    auto *Shl0 = cast<OverflowingBinaryOperator>(Op0);
+    auto *Shl1 = cast<OverflowingBinaryOperator>(Op1);
+
+    if ((!IsSigned && Shl0->hasNoUnsignedWrap() && Shl1->hasNoUnsignedWrap()) ||
+        (IsSigned && Shl0->hasNoSignedWrap() && Shl1->hasNoSignedWrap())) {
+      Constant *One = ConstantInt::get(X->getType(), 1);
+      // Only preserve the nsw flag if dividend has nsw
+      // or divisor has nsw and operator is sdiv.
+      Value *Dividend = Builder.CreateShl(
+          One, Y, "shl.dividend",
+          /*HasNUW*/ true, /*HasNSW*/ Shl0->hasNoUnsignedWrap() ||
+                               (IsSigned && Shl1->hasNoUnsignedWrap()));
----------------
dtcxzyw wrote:

```suggestion
          /*HasNUW*/ true, /*HasNSW*/ IsSigned ? (Shl0->hasNoUnsignedWrap() || Shl1->hasNoUnsignedWrap()) : Shl0->hasNoSignedWrap());
```
Please add more tests about flag preserving.


https://github.com/llvm/llvm-project/pull/68863