Avoid some memory writes on jr in jit

Should improve tight mips function-calling loops a bit.  Probably could apply to arm the same way.

Also, I couldn't divine a clean way to check for the syscall op from the tables (I didn't think it was worth a dedicated info flag, there aren't many left...), so I just checked the encoding directly (pretty sure it's right.)  If there's a better way, I can change it to that.

-[Unknown]