mesa/st: Add support for SPIR-V shaders

mesa/st: Add support for SPIR-V shaders

The SPIR-V codepath uses NIR linking, so we have to preprocess after
the linking steps, which makes things slightly different than GLSL.
To make more clear when the preprocess is happening, I've ended up
inlining st_nir_get_mesa_program() into its caller.

The goal was to make both GLSL and SPIR-V to use the same preprocess
function, the exceptions are:

- SPIR-V codepath don't support NIR state slots yet;
- GLSL lowers shared memory early, so we don't do the deref lowering
  for those.

For now I didn't bother to rename other functions and files (now that
many of them apply to both GLSL and SPIR-V), but we should do this in
further patches.

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