nir/lower_tex: add lowering for texture gradient on cube maps

nir/lower_tex: add lowering for texture gradient on cube maps

This is ported from the Intel lowering pass that we use with GLSL IR.
The NIR pass only handles cube maps, not shadow samplers, which are
also lowered for gen < 8 on Intel hardware. We will add support for
that in a later patch, at which point we should be able to remove
the GLSL IR lowering pass.

v2:
- added a helper to retrieve ddx/ddy parameters (Ken)
- No need to make size.z=1.0, we are only using component x anyway (Iago)

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

v4:
- When emitting the textureLod operation, copy all texture parameters
  from the original textureGrad() (except for ddx/ddy) using a loop
- Add a 'continue' statement if the lowering makes progress because it
  replaces the original texture instruction

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