This adds a new optimization phase to the TurboFan pipeline, which walks
over the effect chain and tries to eliminate redundant loads (and even
some stores) of object fields. We currently ignore element access, but
that will probably need to be handled as well at some point. We also
don't have any special treatment to properly track object maps, which is
also on the list of things that will happen afterwards.

The implementation is pretty simple currently, and probably way to
inefficient. It's meant to be a proof-of-concept to iterate on.

R=jarin@chromium.org
BUG=v8:4930,v8:5141

Review-Url: https://codereview.chromium.org/2120253002
Cr-Commit-Position: refs/heads/master@{#37528}