Auto merge of rust-lang#66326 - Nadrieril:refactor-intrange, r=varkor

Refactor integer range handling in the usefulness algorithm

Integer range handling had accumulated a lot of debt. This cleans up a lot of it.

In particular this:
- removes unnecessary conversions between `Const` and `u128`, and between `Constructor` and `IntRange`
- clearly distinguishes between on the one hand ranges of integers that may or may not be matched exhaustively, and on the other hand ranges of non-integers that are never matched exhaustively and are compared using Const-based shenanigans
- cleans up some overly complicated code paths
- generally tries to be more idiomatic.

As a nice side-effect, I measured a 10% perf increase on `unicode_normalization`.

There's one thing that I feel remains to clean up: the [overlapping range check](rust-lang#64007), which is currently quite ad-hoc. But that is intricate enough that I'm leaving it out of this PR.

There's also one little thing I'm not sure I understand: can `try_eval_bits` fail for an integer constant value in that code ? What would that mean, and how do I construct a test case for this possibility ?

Author: bors