Native context independent code generation should, at the moment, not
use any collected feedback.

We implement this by returning InsufficientFeedback from the heap
broker's ReadFeedbackForX methods if currently compiling nci code.
Thus all feedback.IsInsufficient() calls inside the compiler will
return true (disabling feedback-based optimizations).
FeedbackSource::IsValid() (used in generic lowering) can still return
true.

Bug: v8:8888
Change-Id: I198b6457276073e7376c777b206c50726f1b3645
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2284494
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68726}