[Reassociate] prevent partial undef negation replacement

As shown in the examples in issue #57683, we allow matching
vectors with poison (undef) in this transform (and possibly more),
but we can't then use the partially defined value as a replacement
value in other expressions blindly.

This seems to be avoided in simpler examples of reassociation,
and other passes should be able to clean up the redundant op
seen in these tests.