[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