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``
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