mesa/st: Extract preprocessing NIR steps

mesa/st: Extract preprocessing NIR steps

Refactor to split the glsl_to_nir conversion from the preprocessing
NIR passes into separate functions, so we can use them in SPIR-V.
Unlike in GLSL, there we'll need to perform a few passes with the NIR
linker before doing the individual preprocess calls.

No behavior should change with this patch.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>