Shuffle parts of the register allocator around a bit #1546
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Mostly one can assign GP/FP regs in isolation. However, because x64 requires some FP operations to use a temporary GP register, we currently find ourselves in the uncomfortable position where we have to `push` a GP register in order to be certain that we aren't accidentally clobbering something. This commit introduces a new function in the API `assign_regs` which specifies FP and GP registers in one go. Although we don't take advantage of this yet, it will allow us (one day!) to safely deal with the "I need a temporary GP register" case.
Some queries the codegen and register allocator need to make are "reverse analysis" queries. This commit makes clearer that these are queries on `rev_an` rather than the register allocator. Doing this means allowing another public(ish) field, which looks ugly, but at least makes clearer what the "real" structure is. I increasingly think we need to think of a "proper" fix for this, but our existing structure is unfortunate in several different places, and I can't immediately see an easy migration strategy.
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This PR tidies up some parts of the register allocator (and those bits that use it) so that future work has a slightly more satisfactory base to work upon. There is still a lot more tidying up that we could do here (and, as is not uncommon, this PR exposes more things we should tackle in the future), but this is better than nothing.