This moves the optimization for variables loads targeting lookup slots
in DYNAMIC_GLOBAL and DYNAMIC_LOCAL mode into the AstGraphBuilder. This
way we implicitly get all optimizations that target global loads and
context loads for free.

R=bmeurer@chromium.org
BUG=v8:4513
LOG=n

Review URL: https://codereview.chromium.org/1424943008

Cr-Commit-Position: refs/heads/master@{#31713}