[LoopSimplify] Reduce amount of redundant SCEV invalidation (NFCI)

[LoopSimplify] Reduce amount of redundant SCEV invalidation (NFCI)

We are simplifying the loop and all its children. Each time, we
invalidate the top-most loop. The top-most loop is going to be
the same every time. The cost of SCEV invalidation is largely
independent from how data about the loop is actually cached, so
we should avoid redundant invalidations.