Turn the LoadElimination into a proper graph Reducer so that it can run
together with ValueNumbering and RedundancyElimination to a fixpoint
for maximum load/check elimination. This also adds initial support for
eliminating redundant LoadElement/StoreElement nodes.

BUG=v8:4930,v8:5141

Review-Url: https://codereview.chromium.org/2164253002
Cr-Commit-Position: refs/heads/master@{#38015}