float to/from bits and classify: update for float semantics RFC#128598
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With rust-lang/rfcs#3514 having been accepted, it is clear that hardware which e.g. flushes subnormal to zero is just non-conformant from a Rust perspective -- this is a hardware bug, or maybe an LLVM backend bug (where LLVM doesn't lower floating-point ops in a way that they have the standardized behavior). So update the comments here to make it clear that we don't have to do any of this, we're just being nice.
Also remove the subnormal/NaN checks from the (unstable) const-version of to/from-bits; they are not needed since we decided with the aforementioned RFC that it is okay to get a different result at const-time and at run-time.
r? @workingjubilee since I think you wrote many of the comments I am editing here.