Fix Windows build without dxsdk (#7070.)
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.