glsl/nir: Add and use a gl_nir_link() function

glsl/nir: Add and use a gl_nir_link() function

Perform all the NIR linking steps in order.  Change iris and i965 to
use it.  Suggested by Alejandro.

v2: Add gl_nir_linker_options struct.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com> [v1]