mesa/glspirv: Set separate_shader on shader_info

mesa/glspirv: Set separate_shader on shader_info

The value is copied from the gl_program. If we don’t do this then it
will get reset back to zero in brw_shader_gather_info. This isn’t a
problem for GLSL because in that case the nir_shader is initialised
with a copy of the shader_info from the gl_program.

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