We make assumptions that baseline code exists if we run the optimized code
(e.g., to deopt to the baseline code). If the baseline code has been
cleared by code flushing (only full-codegen) then it might not exist
but there is still optimized code in the map.

BUG=v8:6389

Change-Id: Id4db664afee96c2da3a36a177f425293aae9a0a3
Reviewed-on: https://chromium-review.googlesource.com/503010Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45278}