[llvm] [InstCombine] Fold ((X << nuw Z) binop nuw Y) >>u Z --> X binop nuw (Y >>u Z) (PR #88193)

[Yingwei Zheng via llvm-commits]

================
@@ -1259,6 +1259,53 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
       match(Op1, m_SpecificIntAllowPoison(BitWidth - 1)))
     return new ZExtInst(Builder.CreateIsNotNeg(X, "isnotneg"), Ty);
 
+  // ((X << nuw Z) sub nuw Y) >>u exact Z --> X sub nuw (Y >>u exact Z),
+  // ONLY if I is exact, and both the shift and sub are nuw
+  Value *Y;
+  if (I.isExact() &&
+      match(Op0, m_OneUse(m_NUWSub(m_NUWShl(m_Value(X), m_Specific(Op1)),
+                                   m_Value(Y))))) {
+    Value *NewLshr = Builder.CreateLShr(Y, Op1, "", /* isExact */ true);
----------------
dtcxzyw wrote:

```suggestion
    Value *NewLshr = Builder.CreateLShr(Y, Op1, "", /*isExact=*/true);
```


https://github.com/llvm/llvm-project/pull/88193