We always built a new Phi node for the delegate merge, which is
incorrect when the target block's exception is already a Phi. Use
CreateOrMergeIntoPhi instead.

R=clemensb@chromium.org

Bug: v8:11472
Change-Id: I8af3ab07d536ddfe247ace04cc844207d32adb99
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2707167Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72874}