Only process each LiveRangeBundle once in AssignSpillSlots().

Previously we would try to merge a LiveRangeBundle as many times as
there are LiveRanges inside it. Even though the merge would only happen
once, we would still iterate over all LiveRanges and do expensive checks
for each iteration.

R=sigurds@chromium.org

Bug: v8:11237
Change-Id: I9e613aaf5e571d4c28486dd2c20154336c533563
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2584956
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71741}