[llvm] [DemandedBits] Support non-constant shift amounts (PR #148880)

[Yingwei Zheng via llvm-commits]

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@@ -197,6 +218,20 @@ void DemandedBits::determineLiveOperandBits(
         // (they must be zero).
         if (cast<LShrOperator>(UserI)->isExact())
           AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
+      } else {
+        ComputeKnownBits(BitWidth, UserI->getOperand(1), nullptr);
+        unsigned Min = Known.getMinValue().getLimitedValue(BitWidth - 1);
+        unsigned Max = Known.getMaxValue().getLimitedValue(BitWidth - 1);
+        // Suppose AOut == 0b0000 1001
+        // [min, max] = [1, 3]
+        // shift by 1 we get 0b0001 00100
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dtcxzyw wrote:

```suggestion
        // shift by 1 we get 0b0001 0010
```

https://github.com/llvm/llvm-project/pull/148880