vkd3d-shader: Introduce a COMPOSITE_CONSTRUCT vsir instruction part 1.

Use of temps will become messy when we add native 16-bit types, so constructing SSA composites for those will save verbosity in the SPIR-V output. It also can be used for constructing coordinates, offsets and texels in place of the current scheme of writing each temp component. Reducing the SPIR-V code size is useful for shader caching, as the cache can become very large.