[LoopNest] Handle loop-nest passes in LoopPassManager

Per http://llvm.org/OpenProjects.html#llvm_loopnest, the goal of this
patch (and other following patches) is to create facilities that allow
implementing loop nest passes that run on top-level loop nests for the
New Pass Manager.

This patch extends the functionality of LoopPassManager to handle
loop-nest passes by specializing the definition of LoopPassManager that
accepts both kinds of passes in addPass.

Only loop passes are executed if L is not a top-level one, and both
kinds of passes are executed if L is top-level. Currently, loop nest
passes should have the following run method:

PreservedAnalyses run(LoopNest &, LoopAnalysisManager &,
LoopStandardAnalysisResults &, LPMUpdater &);

Reviewed By: Whitney, ychen
Differential Revision: https://reviews.llvm.org/D87045