Having a NoOpLoopNestPass can ensure that only outermost loop is invoked
for a LoopNestPass with a lit test.
There are some existing passes that are implemented as LoopNestPass, but
they are still using LOOP_PASS macro.
It would be easier to identify LoopNestPasses with a LOOPNEST_PASS
macro.
Differential Revision: https://reviews.llvm.org/D113185
Whitney Tsang