The only reason why this hasn't been causing us problems before is
because the only use of ineg in an iadd with a carry-out is ineg64 and
that's basically guaranteed to be folded in NIR because it's unary.
However, if we want to be able to use SrcMod::INeg more, we need to be
more careful about it.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30402>
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