Off-heap code may have pc values that are not within
[instruction_start, instruction_end]. Handle these kinds of objects in
heap Code lookup functions.

Bug: v8:6666
Change-Id: Ib7a58f962ef6311c48234e506a35485fd5966814
Reviewed-on: https://chromium-review.googlesource.com/934184
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51518}