x86jit: Sketch some initial SIMD apis.
x86jit: Add force-non-simd to all current ops.

Unless they already use MapRegs, because that will automatically handle
it.
x86jit: Add fallbacks when moving from VS -> V.
x86jit: Add basic mapping of SIMD regs.

Not tested yet, just sketched out.  All very suboptimal.
x86jit: Add some typesafety.
x86jit: Add an invariant contract to the fpu cache.

This should help catch things better in debug mode.
x86jit: Add a TryMapDirtyInInVS() for 3-op.
x86jit: Initial simd for VecDo3().  Broken.

I'm not sure why/where it's broken...
x86jit: Split out the logic for loading simd regs.
x86jit: Add some helpers to check state.
x86jit: Add a check to see if we can map simd.
x86jit: Load sequential regs in one shot.
x86jit: Optimize a 2-reg simd load.