CL: Use uniform variables instead of shader_in for kernel inputs

This is just a mechanical replace. The semantics of inputs for CL are a closer match to the semantics of uniforms for graphics. Rather than cross-stage data, it's data that every thread sees uniformly.

This also makes it simpler to deal with images (!5242 (merged)): currently the SPIR-V to NIR converter generates tex and image intrinsics by casting images/samplers to uniform derefs.

/cc @jekstrand @karolherbst

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