In previous change https://crrev.com/c/2274308 , I attempted to fix an
issue where FindOptimalSpillingPos could sometimes fail to find the
LiveRange that covers the top of the loop. However, I misunderstood how
TopLevelLiveRange::GetChildCovers behaves, so I introduced a different
case where FindOptimalSpillingPos would fail to find the right
LiveRange. This change updates GetChildCovers to do what I had thought
it would do, so it can find the right LiveRange in all cases.

     chromium:1102243

Bug: chromium:1101958, chromium:1101954, chromium:1102257,
Change-Id: If91c642c3f7f5e3a8b4cfaa3b3577865c84afcb6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2288660Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#68758}