[InstCombine] Fold fneg over select (#89947)

As we folds fabs over select in
https://github.com/llvm/llvm-project/pull/86390, this patch folds fneg
over select to make sure nabs idioms are generated.
Addresses
https://github.com/llvm/llvm-project/pull/86390#discussion_r1568862289.

Alive2 for FMF propagation: https://alive2.llvm.org/ce/z/-h6Vuo