Generalize LCSSA to handle loops with exits with predecessors outside

the loop. This is needed because with indirectbr it may not be possible
for LoopSimplify to guarantee that all loop exit predecessors are
inside the loop. This fixes PR5437.

LCCSA no longer actually requires LoopSimplify form, but for now it
must still have the dependency because the PassManager doesn't know
how to schedule LoopSimplify otherwise.

llvm-svn: 86569