x86jit: Add basic support for mapping SIMD

This implements only the VecDo3 ops using SIMD.  It seems to work from the games I've tested.

I'd like to simplify though.  I'm thinking it'd be better to change this pattern:

```c++
	VectorSize sz = GetVecSize(op);
	int n = GetNumVectorElements(sz);

	u8 sregs[4], tregs[4], dregs[4];
	GetVectorRegsPrefixS(sregs, sz, _VS);
	GetVectorRegsPrefixT(tregs, sz, _VT);
	GetVectorRegsPrefixD(dregs, sz, _VD);

	if (fpr.TryMapDirtyInInVS(dregs, sz, sregs, sz, tregs, sz)) {
		...

		ApplyPrefixD(dregs, sz);
		fpr.ReleaseSpillLocks();
		return;
	}

	// Flush SIMD.
	fpr.SimpleRegsV(sregs, sz, 0);
	fpr.SimpleRegsV(tregs, sz, 0);
	fpr.SimpleRegsV(dregs, sz, MAP_NOINIT | MAP_DIRTY);

	...

	ApplyPrefixD(dregs, sz);

	fpr.ReleaseSpillLocks();
```

To something more like:

```c++
	// Things like: VECREGS_D_LOAD, VECREGS_DSZ_HALF, VECREGS_NO_PREFIX, etc...
	VecOpRegs regs = GetVectorRegs(op, VECREGS_DST);

	if (fpr.TryMapVS(regs)) {
		...

		// Or maybe done automatically if mapped?  Just worried about DISABLE.
		regs.FinalizeD();
		return;
	}

	fpr.SimpleRegsV(regs);

	...

	regs.FinalizeD();
```

And also matrices... those are complicated...

-[Unknown]
Looks like you do write some stuff back? Isn't it enough to mark the lane as discarded, do you really need to flush the others?