Auto merge of #130220 - RalfJung:float-classify, r=workingjubilee

simplify float::classify logic

I played around with the float-classify test in the hope of triggering x87 bugs by strategically adding `black_box`, and still the exact expression `@beetrees` suggested [here](rust-lang/rust#129835 (comment)) remains the only case I found where we get the wrong result on x87. Curiously, this bug only occurs when MIR optimizations are enabled -- probably the extra inlining that does is required for LLVM to hit the right "bad" case in the backend. But even for that case, it makes no difference whether `classify` is implemented in the simple bit-pattern-based version or the more complicated version we had before.

Without even a single testcase that can distinguish our `classify` from the naive version, I suggest we switch to the naive version.

Author: bors