In previous PRs, I changed the match desugaring to generate more efficient code for ints/chars and the like. But this doesn't help when you're matching strings, ranges, or other crazy complex things (leading to #29740). This commit restructures match desugaring *yet again* to handle that case better -- basically we now degenerate to an if-else-if chain in such cases.
~~Note that this builds on https://github.com/rust-lang/rust/pull/29763 which will hopefully land soon. So ignore the first few commits.~~ landed now
r? @Aatch since he's been reviewing the other commits in this series
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