[InstCombine] fix nsz (fast-math) propagation from fneg-of-select

As discussed in the post-commit comments for:
3cdd05e5

It seems to be safe to propagate all flags from the final fneg
except for 'nsz' to the new select:
https://alive2.llvm.org/ce/z/J_APDc

nsz has unique FMF semantics: it is not poison, it is only
"insignificant" in the calculation according to the LangRef.