[llvm] [SelectionDAG] Make sure demanded lanes for AND/MUL-by-zero are frozen (PR #180727)

[Simon Pilgrim via llvm-commits]

================
@@ -3806,19 +3806,22 @@ bool TargetLowering::SimplifyDemandedVectorElts(
     if (SimplifyDemandedVectorElts(Op1, DemandedElts, SrcUndef, SrcZero, TLO,
                                    Depth + 1))
       return true;
-    // If we know that a demanded element was zero in Op1 we don't need to
-    // demand it in Op0 - its guaranteed to be zero.
-    APInt DemandedElts0 = DemandedElts & ~SrcZero;
-    if (SimplifyDemandedVectorElts(Op0, DemandedElts0, KnownUndef, KnownZero,
+    // FIXME: If we know that a demanded element was zero in Op1 we don't need
+    // to demand it in Op0 - its guaranteed to be zero. There is however a
+    // restriction, as we must not make any of the originally demanded elements
+    // more poisonous. We could reduce amount of elements demanded, but then we
+    // also need a to inform SimplifyDemandedVectorElts that some elements must
+    // not be made more poisonous.
----------------
RKSimon wrote:

How about:
```
APInt DemandedElts0 = DemandedElts;
if (isGuaranteedNotToBePoison(Op0, SrcZero))
  DemandedElts0 = DemandedElts & ~SrcZero;
```

https://github.com/llvm/llvm-project/pull/180727