When Turbofan gets triggered during bootstrapping (via --always-opt), some
context fields still hold undefined rather than their actual value.

Bug: v8:7790
Change-Id: Id87c593182fa8450ba9415d144d105281e48236f
Reviewed-on: https://chromium-review.googlesource.com/c/1268240Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56449}