nir: add a pass for lowering (un)pack_double_2x32

nir: add a pass for lowering (un)pack_double_2x32

v2: Undo unintended change to the signature of
    nir_normalize_cubemap_coords (Iago).

v3: Move to compiler/nir (Iago)

v4: Remove Authors from copyright header (Michael Schellenberger)

v5 (Sam):
- Use nir_channel() and nir_ssa_for_alu_src() helpers (Jason)
- Inline lower_double_pack_instr() code into lower_double_pack_block()
  (Jason).
- Initialize nir_builder at lower_double_pack_impl() (Jason).

Signed-off-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>