[NPM] Properly reset parent loop after loop passes
This fixes https://bugs.llvm.org/show_bug.cgi?id=49185
When NDEBUG is not set, LPMUpdater checks if the added loops have the same parent loop as the current one in addSiblingLoops.
If multiple loop passes are executed through LoopPassManager, U.ParentL will be the same across all passes.
However, the parent loop might change after running a loop pass, resulting in assertion failures in subsequent passes.
This patch resets U.ParentL after running individual loop passes in LoopPassManager.
Reviewed By: asbirlea, ychen
Differential Revision: https://reviews.llvm.org/D96727