The SharedFeedbackSlot helper class allow bytecodes to share one feedback
slot. The helper will only create the slot on-demand, at the first-use.
This does not encapsulate the use-case of FeedbackSlotCache.

Change-Id: I22aec19d59e52e7395898fa2a59c5c1ec95abbe8
Reviewed-on: https://chromium-review.googlesource.com/1189904
Commit-Queue: Hai Dang <dhai@google.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55452}