microsoft/compiler: Use texture types instead of sampler types for textures

This adds some complexity to the lowering process for NIR before sending it to DXIL, but the resulting NIR is much easier to reason about. Any type that is a sampler will be a "bare" sampler, which matches what D3D/DXIL expects from a sampler. Anything that's a texture is what D3D/DXIL expects as a shader resource, and images are read-write resources.