Optimize GetPrototype

This introduces Hydrogen for %_GetPrototype. The code falls back on
runtime if the object needs access checks or if its prototype is a
hidden prototype.

BUG=None
LOG=Y
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/756423006

Cr-Commit-Position: refs/heads/master@{#25694}