jit: Discard unused regs before a syscall

[unknownbrackets]

This is a pretty minor optimization, though.

-[Unknown]

[hrydgard]

Seems fine, but I'm worried about even more of these issues where games rely on the contents of registers (or stack, but that's separate), violating the calling convention. Discarding these would seem to reduce the likelyhood of getting the "right" values in the registers by a bit...

[unknownbrackets]

Well, right after the syscall we will fill the values with 0xDEADBEEF anyway (which is what the system does, at least for kernel funcs / actual syscalls.) The only reg this is not true for is V1, but that's only if the syscall doesn't use it. I can remove V1. I guess V0 also if there are any void funcs.

-[Unknown]

[hrydgard]

Right, thanks for the 0xDEADBEEF reminder.. drop V0 and V1 and I'll merge.

[unknownbrackets]

Okay, done. Also added HI/LO (these are also filled with DEADBEEF.)

-[Unknown]