Previously isAlocatable was updated to allow inheritance from any
superclass for a generated register class, but other properties are
still inherited from its nearest superclass. This could cause
a generated regclass inherit undesired properties, e.g., tsflags, from
an unallocatable superclass due to the topological inheritance order.
This change updates to inherit properties from the nearest allocatable
superclass if possible and includes a test to demonstrate a potential
incorrect inheritance of tsflags.
a074831cd9