[llvm] [InstCombine] Fold `(X * (Y << K)) u>> K -> X * Y` when highbits are not demanded (PR #111151)

[via llvm-commits]

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@@ -770,6 +770,15 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Instruction *I,
             return InsertNewInstWith(Shl, I->getIterator());
           }
         }
+
+        const APInt *Factor;
+        if (match(I->getOperand(0),
+                  m_OneUse(m_Mul(m_Value(X), m_APInt(Factor)))) &&
+            Factor->countr_zero() >= ShiftAmt) {
+          BinaryOperator *Mul = BinaryOperator::CreateMul(
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goldsteinn wrote:

You can preserve `nuw` and `nsw` iff you have `nuw`:
https://alive2.llvm.org/ce/z/P9NudG

https://github.com/llvm/llvm-project/pull/111151