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Air: add explicit
repeat
instruction to repeat loops
This commit introduces a new AIR instruction, `repeat`, which causes control flow to move back to the start of a given AIR loop. `loop` instructions will no longer automatically perform this operation after control flow reaches the end of the body. The motivation for making this change now was really just consistency with the upcoming implementation of ziglang#8220: it wouldn't make sense to have this feature work significantly differently. However, there were already some TODOs kicking around which wanted this feature. It's useful for two key reasons: * It allows loops over AIR instruction bodies to loop precisely until they reach a `noreturn` instruction. This allows for tail calling a few things, and avoiding a range check on each iteration of a hot path, plus gives a nice assertion that validates AIR structure a little. This is a very minor benefit, which this commit does apply to the LLVM and C backends. * It should allow for more compact ZIR and AIR to be emitted by having AstGen emit `repeat` instructions more often rather than having `continue` statements `break` to a `block` which is *followed* by a `repeat`. This is done in status quo because `repeat` instructions only ever cause the direct parent block to repeat. Now that AIR is more flexible, this flexibility can be pretty trivially extended to ZIR, and we can then emit better ZIR. This commit does not implement this. Support for this feature is currently regressed on all self-hosted native backends, including x86_64. This support will be added where necessary before this branch is merged.
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