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]
Well, some cases right now are:
1. Overlap, but different order, in D, S, or T.
2. Any affected regs already mapped in a VS reg, but with alternate order
3. Any affected regs already mapped in a VS reg, but with alternate length(s).

It is possible to force it, for which I have a MapRegsVS(), but my supposition is that in the second case above, the pain of reordering (stores, loads, shuffles), especially multiple times, may be worse than the non-simd approach.  That being said, it may be better for force it, not sure.

Case 3 above seems optimizable (especially for the < case), but not implemented yet (a primary reason for Try, so we can add features later.)

Case 1 is complex.  It's probably solvable, but it will mean mapping xrs one-way, possibly by shifting their locations to free temp regs.  As long as there's only one destination that may be safe.  But I'm not sure.  Anyway, not implemented yet.

-[Unknown]