nir: Add a bit_size to nir_register and nir_ssa_def

nir: Add a bit_size to nir_register and nir_ssa_def

This really hacky commit adds a bit size to registers and SSA values.  It
also adds rules in the validator to validate that they do the right things.

It's still an open question as to whether or not we want a bit_size in
nir_alu_instr or if we just want to let it inherit from the destination.
I'm inclined to just let it inherit from the destination.  A similar
question needs to be asked about intrinsics.

v2 (Connor):
  - Relax validation: comparisons have explicit destination sizes
    and implicit source sizes.

v3 (Sam):
- Use helpers to get size and base types of nir_alu_type enum.

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