[turbofan] Add poor man's store elimination for storing to fields.

[turbofan] Add poor man's store elimination for storing to fields.

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

Committed: https://crrev.com/f190a6d23d6bc7cd17c54ad06e8e5924751e7e72
Cr-Commit-Position: refs/heads/master@{#27697}

[Benedikt Meurer, 2015-04-09 09:57:58]

PTAL

[dcarney, 2015-04-09 09:58:54]

lgtm

[Benedikt Meurer, 2015-04-09 10:29:38]

Committed patchset #1 (id:1) manually as
f190a6d23d6bc7cd17c54ad06e8e5924751e7e72 (presubmit successful).

[commit-bot: I haz the power, 2015-04-09 10:29:39]

Patchset 1 (id:??) landed as
https://crrev.com/f190a6d23d6bc7cd17c54ad06e8e5924751e7e72
Cr-Commit-Position: refs/heads/master@{#27697}