Adds a fast path for loading DYNAMIC_GLOBAL variables, which are lookup
variables that can be globally loaded, without calling the runtime, as long as
there was no context extension by a sloppy eval along their context chain.

BUG=v8:5263

Review-Url: https://codereview.chromium.org/2347143002
Cr-Commit-Position: refs/heads/master@{#39537}