We already have an optimization to constant-fold access to an object's
prototype via the special __proto__ accessor (specified in appendix B).
We can use the same optimization to also constant-fold accesses to an
object's prototype via the official Object.getPrototypeOf function.

Also add the optimization for Reflect.getPrototypeOf, which is
equivalent for object inputs.

This is commonly used by Babel to implement various new language
features, for example subclassing and certain property lookups.

R=yangguo@chromium.org
BUG=v8:6292

Review-Url: https://codereview.chromium.org/2841463002
Cr-Commit-Position: refs/heads/master@{#44788}