The compilation logic never used the saved FeedbackVector for Script
compiles when looking up the CompilationCache, so remove it and
simplify the return value of LookupScript to be a
MaybeHandle<SharedFunctionInfo>

Change-Id: Ib1d833f997b299e2e79621bd8509bdfd911d4e10
Reviewed-on: https://chromium-review.googlesource.com/924002
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51443}