To be able to lazily allocate BlockScopes, we need to reduce access to scope(). The explicit accesses removed here just need access to the current scope to allocate unresolved variable proxies. Those in the future should go over ScopeState rather than directly to Scope.

BUG=v8:5209

Review-Url: https://codereview.chromium.org/2224843003
Cr-Commit-Position: refs/heads/master@{#38456}