This adds handling of JSLoadDynamicContext nodes to JSTypedLowering to
perform extension checks and an inline fast path. The fast path is a
context slot load targeting a specific context.

R=bmeurer@chromium.org
BUG=v8:4131
LOG=N

Review URL: https://codereview.chromium.org/1155543003

Cr-Commit-Position: refs/heads/master@{#28823}