In {MergeFullStackWith}, we were accidentally looking at the cached
instance in the current {cache_state_} instead of the state passed as
{source}. This could lead to missing reload of the instance after a
conditional branch.

R=thibaudm@chromium.org

Bug: chromium:1179182
Change-Id: Ida3c06491f7973a183c43745159abbf6aa8a058b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2704081Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72864}