Rollup merge of #121391 - Nadrieril:fix-liveness, r=compiler-errors

never patterns: Fix liveness analysis in the presence of never patterns

There's a bunch of code that only looks at the first alternative of an or-pattern, under the assumption that all alternatives have the same set of bindings. This is true except for never pattern alternatives  (e.g. `Ok(x) | Err(!)`), so we skip these. I expect there's other code with this problem, I'll have to check that later.

I don't have tests for this yet because mir lowering causes other issues; I'll have some in the next PR.

r? ``@compiler-errors``

Author: matthiaskrgr