This loads references to {null} values from the instance object instead
of embedding them into the generated code. It is one step towards making
the {WasmCode} objects independent of the Isolate.

Note that this also fixes an issue with the serializer/deserializer that
failed to properly serialize {null} values and accidentally collapsed
them to {undefined} values instead.

R=ahaas@chromium.org
TEST=mjsunit/regress/wasm/regress-7785
BUG=v8:7424,v8:7785

Change-Id: Ie436c2d96890e7c8c89ffe2bd4189a759254775b
Reviewed-on: https://chromium-review.googlesource.com/1070981
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53352}