JSInliner class wrongly assumed that all functions passing through
JSInliningHeuristic have feedback vectors, but that's not the case
when the inlining candidate hasn't been called yet.

Bug: chromium:961522
Change-Id: I89c0f2098add19d9b59394f1e7230cbec426119d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1605720Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61400}