nir: Add explicitly sized types

nir: Add explicitly sized types

v2: Fix size/type mask to properly handle 8-bit types.

v3: Add helpers to get the bitsize and base type of a
nir_alu_type enum.

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