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]
Hm. Here we know that reg will be mapped to a register, right? Then it should be OK to simply do MULSS(reg, M(&vavg_table[n])); to save the MOV?