[turbofan] Optimized lowering of DYNAMIC_GLOBAL lookup slot loads.

[turbofan] Optimized lowering of DYNAMIC_GLOBAL lookup slot loads.

This adds handling of JSLoadDynamicGlobal nodes to JSTypedLowering to
perform extension checks and an inline fast path. The fast path is a
global variable load from the global object.

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

Review URL: https://codereview.chromium.org/1150723005

Cr-Commit-Position: refs/heads/master@{#28750}