Noah Goldstein 9ef829097b [InstCombine] Fix buggy transform in foldNestedSelects; PR 71330 ...

The bug is that `IsAndVariant` is used to assume which arm in the
select the output `SelInner` should be placed but match the inner
select condition with `m_c_LogicalOp`. With fully simplified ops, this
works fine, but its possible if the select condition is not
simplified, for it match both `LogicalAnd` and `LogicalOr` i.e `select
true, true, false`.

In PR71330 for example, the issue occurs in the following IR:
```
define i32 @bad() {
  %..i.i = select i1 false, i32 0, i32 3
  %brmerge = select i1 true, i1 true, i1 false
  %not.cmp.i.i.not = xor i1 true, true
  %.mux = zext i1 %not.cmp.i.i.not to i32
  %retval.0.i.i = select i1 %brmerge, i32 %.mux, i32 %..i.i
  ret i32 %retval.0.i.i
}
```

When simplifying:
```
%retval.0.i.i = select i1 %brmerge, i32 %.mux, i32 %..i.i
```

We end up matching `%brmerge` as `LogicalAnd` for `IsAndVariant`, but
the inner select (`%..i.i`) condition which is `false` with
`LogicalOr`.

Closes #71489

2023-11-09 16:36:49 -06:00

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