nir: Plumb the shader stage into glsl_to_nir().

nir: Plumb the shader stage into glsl_to_nir().

The next commit needs to know the shader stage in glsl_to_nir().
To facilitate that, we pass the gl_shader rather than the raw exec_list
of instructions.  This has both the exec_list and the stage.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>