x86jit: Set CTXREG in a fixed place.
jit: Use nicknames for a few more static regs.
x86jit: Skip reload of EAX in safe cases.

Probably no real improvement in perf, but a bit cleaner and one less TODO.
Merge pull request #7191 from unknownbrackets/jit-minor

x86jit: Minor tweaks to asm dispatcher