Tablgen refactoring

[Sean Silva]

I think an InitVisitor of some kind might be independently useful, but
this particular issue would likely be better solved like you did in
you other patch for baseClassOf (i.e. remove the overloads and have
just one method taking the base class (Init*) that tests the
convertibility condition with regular code instead of using
overloading (of a virtual!) to implement that logic). At least for now
that will probably be the simplest approach (and it reduces the
dependency graph between Init subclasses to only the edges of
convertibility, rather than a complete graph).

-- Sean Silva