[InstCombine] try harder to preserve 'nsz' in fneg-of-select transform

The corner case where 'nsz' needs to be removed is very narrow
as discussed here:
https://reviews.llvm.org/rG3cdd05e519dd

If the select condition is not undef, there's no problem with
propagating 'nsz':
https://alive2.llvm.org/ce/z/4GWJdq