I have a need to have SwitchEnum{,Addr}Inst have different base classes
(TermInst, OwnershipForwardingTermInst). To do this I need to add a template to
SwitchEnumInstBase so I can switch that BaseTy. Sadly since we are using
SwitchEnumInstBase as an ADT type as well as an actual base type for
Instructions, this is impossible to do without introducing a template in a ton
of places.
Rather than doing that, I changed the code that was using SwitchEnumInstBase as
an ADT to instead use a proper ADT SwitchEnumBranch. I am happy to change the
name as possible see fit (maybe SwitchEnumTerm?).
