[llvm] [SelectionDAG] Fix incorrect fold condition in foldSetCCWithFunnelShift. (PR #137637)

[Jay Foad via llvm-commits]

================
@@ -4462,7 +4462,8 @@ static SDValue foldSetCCWithFunnelShift(EVT VT, SDValue N0, SDValue N1,
 
   unsigned BitWidth = N0.getScalarValueSizeInBits();
   auto *ShAmtC = isConstOrConstSplat(N0.getOperand(2));
-  if (!ShAmtC || ShAmtC->getAPIntValue().uge(BitWidth))
+  APInt AmtVal = ShAmtC->getAPIntValue();
+  if (!ShAmtC || AmtVal.uge(BitWidth) || AmtVal.isZero())
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jayfoad wrote:

A more precise way to implement this check would be to first reduce the shift amount modulo the bitwidth:
```
  unsigned ShAmt = ShAmtC->getAPIntValue().urem(BitWidth).getZExtValue();
```
and then bail out if it is zero.

https://github.com/llvm/llvm-project/pull/137637