Always assert that 'pg' does not have a lane size.

We did this for PgLow8, but not for 4-bit 'pg' fields.

In practice, we plan to relax this in the future, permitting lane sizes
where they match the rest of the instruction, but this patch makes our
checks consistent in the meantime.

Change-Id: Ie791027f217eabab305dbd22b8c0e77926c9d3b8