[InstCombine] fold fnegs around select

This is one of the folds requested in:
https://llvm.org/PR39480

https://alive2.llvm.org/ce/z/NczU3V

Note - this uses the normal FMF propagation logic
(flags transfer from the final value to new/intermediate ops).
It's not clear if this matches what Alive2 implements,
so we may want to adjust one or the other.