If FLAG_enable_embedded_constant_pool is false, the field for the
builtin index would alias the field for the constant pool offset in the
code object. This makes constant_pool() return the builtin index, but it
also makes set_constant_pool() override the builtin index (and vice
versa).
This CL fixes this by making all constant_pool accessors honor that
flag.

R=mstarzinger@chromium.org

Change-Id: I88803a4f28bd5a2fe85a310708c7a365cc457339
Reviewed-on: https://chromium-review.googlesource.com/970586Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52076}