Auto merge of #42431 - nagisa:core-float-2, r=alexcrichton

Fix NaN handling in is_sign_negative/positive

This would be my proposed fix for the #42425 provided we decide it is indeed a problem.

Note this would technically be a breaking change to a stable API. We might want to consider deprecating these methods and adding new ones.

diff --cc src/libcore/num/f32.rs
Simple merge

diff --cc src/libcore/num/f64.rs
Simple merge

diff --cc src/libstd/f32.rs
Simple merge

diff --cc src/libstd/f64.rs
Simple merge