Previously we used a FixedArray for the FeedbackMetadata, packing bits
of information into Smi fields. On 64-bit platforms, we waste at least
half of the available memory by using the Smi representation.

Given that this is just raw data (no pointers), we can just use a new
type that uses the existing packing scheme to store the data in int32
format instead.

This CL changes FeedbackMetadata to a new subclass of HeapObject. This
is to reduce the API surface exposed, in comparison to extending/using
a more general purpose data structure like ByteArray, which is also just
raw data.

FeedbackMetadata only exposes general purpose methods for accessing
slots, but hides the implementation detail of packing bits into int32
fields.

This CL also introduces a sentinal EmptyFeedbackMetadata, because there
are ~750 empty FeedbackMetadata objects when running an empty program in
V8. These are probably for builtins.

Bug: v8:7500
Change-Id: Ic85563153abbd71a22854cee8519260c32b1e9ab
Reviewed-on: https://chromium-review.googlesource.com/945730
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51842}