nir/lower_tex: add lowering for texture gradient on shadow samplers

nir/lower_tex: add lowering for texture gradient on shadow samplers

This is ported from the Intel lowering pass that we use with GLSL IR.
This takes care of lowering texture gradients on shadow samplers other
than cube maps. Intel hardware requires this for gen < 8.

v2 (Ken):
 - Use the helper function to retrieve ddx/ddy
 - Swizzle away size components we are not interested in

v3:
- Get rid of the ddx/ddy helper and use nir_tex_instr_src_index
  instead (Ken, Eric)

v4:
- Add a 'continue' statement if the lowering makes progress because it
  replaces the original texture instruction

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v3)