Auto merge of #116281 - Nadrieril:eager-const-eval, r=cjgillot

Cleanup number handling in match exhaustiveness

Doing a little bit of cleanup; handling number constants was somewhat messy. In particular, this:

- evals float consts once instead of repetitively
- reduces `Constructor` from 88 bytes to 56 (`mir::Const` is big!)

The `fast_try_eval_bits` function was mostly constructed from inlining existing code but I don't fully understand it; I don't follow how consts work and are evaluated very well.

Author: bors