[PM] Now that LoopInfo isn't in the Pass type hierarchy, it is much

cleaner to derive from the generic base.

Thise removes a ton of boiler plate code and somewhat strange and
pointless indirections. It also remove a bunch of the previously needed
friend declarations. To fully remove these, I also lifted the verify
logic into the generic LoopInfoBase, which seems good anyways -- it is
generic and useful logic even for the machine side.

llvm-svn: 226385