x86jit: Implement vfad and vavg.
x86jit: Implement vi2x instructions.

Also, my opcodes were wrong in the test (shifted the pair bit the wrong
way, oops.)

AFAICT, there's no reason PSRAD/etc. were not encoding REX...
x86jit: Micro optimize vi2x a bit with ssse3/sse4.

Both are small wins.
x86jit: Avoid some regcache pollution.
x86jit: Simplify vavg a bit.
x86jit: Oops, can't have an imm here.
x86jit: Micro optimize vi2f a little bit.

This didn't help overall perf much but micro benchmarks are better.
Merge pull request #7071 from unknownbrackets/jit-minor

x86jit: Implement vfad/vavg and vi2x