vkd3d-shader/hlsl: Make single-component swizzles retrieve a scalar and add more swizzle tests.

Vectors cannot be used as array indexes, however, single-component swizzles (such as vec.x) can be used.

This suggests that single-component swizzles should actually be scalars and not vectors of dimx = 1.

It is worth noting that the use of swizzles on scalars should still be allowed.