x86jit: Implement vfad/vavg and vi2x

Now at 1050% in Grand Knights History (+almost 3% from 1020%.)

Micro tests of the sse4.1/ssse3 improvements showed barely a 15% improvement at best, but improved overall perf by over 0.7% (it was just below 1045% before that.)

-[Unknown]
Yeah, that only really applies to the *PS ops, the *SS ops don't really care except for a possible minor penalty, and vavg_table is aligned enough anyway.