Re: [PATCH v5 0/1] MR74: vkd3d-shader/hlsl: Support dot() for SM1.

[Francisco Casas (＠fcasas)]

The following shouldn't affect this MR but I found it worth noting:

When compiling 
```hlsl
uniform float2 a, b;

float4 main() : sv_target
{
    return dot(a, b);
}
```
I see a little discrepancy between how the swizzles are written in native:
```
dp2add r0.x, r0, r1, c2.x
````
versus ours:
```
dp2add r0, r1, r0, r2
```
The [documentation of `dp2add`](https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dp2add---ps) says that the `.x` is a " required replicate swizzle". I have not idea of what that means...

Regardless, that swizzle would appear if we just made sure that `arg3->reg.writemask = 1`, but it seems that we are not setting the writemasks to 1 when allocating scalar types in SM1. This may be a bug.