intrinsics: clarify float min/max behavios for NaNs and signed zeros

[RalfJung]

This makes the currently implemented semantics more explicit, so one does not have to go look for the publicly exposed version of the operation to figure out what exactly should happen.

[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[rustbot]

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[rustbot]

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr