[TableGen] Merge RecTy::typeIsConvertibleTo and RecTy::baseClassOf. NFC

typeIsConvertibleTo was just calling baseClassOf(this) on the argument passed to it, but there weren't different signatures for baseClassOf so passing 'this' didn't really do anything interesting. typeIsConvertibleTo could have just been a non-virtual method in RecTy. But since that would be kind of a silly method, I instead re-distributed the logic from baseClassOf into typeIsConvertibleTo.

llvm-svn: 238648