Enhance CodePlacementOpt's unconditional intra-loop branch elimination logic

to be more general and understand more varieties of loops.

Teach CodePlacementOpt to reorganize the basic blocks of a loop so that
they are contiguous. This also includes a fair amount of logic for preserving
fall-through edges while doing so. This fixes a BranchFolding-ism where blocks
which can't be made to use a fall-through edge and don't conveniently fit
anywhere nearby get tossed out to the end of the function.

llvm-svn: 84295