HLSL: Error with scattered l-value read/modify/write with matrix accessor swizzles

There are errors when trying to do some simple assignments like the following to a matrix:
```
float3x4 anchorTransform;
...
anchorTransform._m03_m13_m23 -= localSpaceOffset.xyz;
```

The error is:
```
error: 'assign' : only simple assignment to non-simple matrix swizzle is supported
```