This is a very simple dead store elimination that removes StoreField
nodes which are immediately followed by other StoreField nodes that
store to the same field. Ideally there should be a fully featured store
elimination, which walks over the effect graph starting from the end,
but there are some technical difficulties to solve before we can get to
that, esp. we need to think about "effect producing" operators like
ValueEffect first. Once we have that, it is trivial to remove this temporary
poor man's store elimination.

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/1070003002

Cr-Commit-Position: refs/heads/master@{#27697}