nir/validate: Rework ALU bit-size rule validation

nir/validate: Rework ALU bit-size rule validation

The original bit-size validation wasn't capable of properly dealing with
instructions with variable bit sizes.  An attempt was made to handle it
by looking at source and destinations but, because the validation was
done in validate_alu_(src|dest), it didn't really have the needed
information.  The new validation code is much more straightforward and
should be more correct.

Reviewed-by: Eric Anholt <eric@anholt.net>