ZJIT: Panic unimplemented for OOB basic block args

[havenwood]

• Go from 8 to 10 ALLOC_REGS registers for ARM64 and x86_64
• Panic about unimplemented register spilling when block args are out of bounds

Testing ZJIT I noticed an index out of bounds panic with:

ruby --zjit -e "pp 0"

Pending the implementation of register spilling, I propose:

1. Improve the panic message when there aren't enough registers for block arguments
2. Increase the number of basic block registers from 8 to 10 to cover many more cases in the interim.

[k0kubun]

r14 and r15 are callee-saved registers. The register allocator currently doesn't support callee-saved registers, so they're deliberately not added to the list at the moment. This branch could cause an undefined behavior depending on a C function called in JIT code. We'd need to preserve them when entering/leaving JIT code, but that's also overhead we need to carefully handle.

So our path forward is probably to support register spill first, before expanding the register pool to callee-saved registers.

[k0kubun]

The panic (by unimplemented!) itself works as an inline test, so I don't think you should test the test. It's also tedious to update the "11/10" part whenever you touch the ALLOC_REGS constant.

[havenwood]

Thank you for explaining why I was in error, @k0kubun. I very much appreciate your thoughtful response. I've updated the PR accordingly, removing the new registers and unnecessary test.

I updated this PR to only include the unimplemented panic message.

[k0kubun]

Thanks 👍