The megamorphic case in the KeyedStoreIC doesn't use/update any feedback.
We could use this path to fast path some of the keyed stores instead of
misssing to the runtime when the feedback vector is not available. This
cl, moves the check for feedback vector from the bytecode handler to the
KeyedStoreIC and calls the KeyedStoreIC_Megamorphic builtin when there is
no valid feedback vector. This will help improve the performance of no
feedback/jitless case.

Bug: v8:8293
Change-Id: I71c128b355d47ac20e50fc836f4bc2cf0aab1154
Reviewed-on: https://chromium-review.googlesource.com/c/1460946
Commit-Queue: Mythri Alle <mythria@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59590}