[InstCombine] try harder to propagate 'nsz' through fneg-of-select

This can be viewed as swapping the select arms:
https://alive2.llvm.org/ce/z/jUvFMJ
...so we don't have the 'nsz' problem with the more general fold.

This unlocks other folds for the motivating fabs example.
This was discussed in issue #38828.