[turbofan] Introduce CommonOperatorReducer.

[turbofan] Introduce CommonOperatorReducer.

The CommonOperatorReducer currently takes care of redundant Phis,
EffectPhis and Selects. This functionality overlaps with ControlReducer,
but is required to make certain optimizations effective, since the
ControlReducer only runs really early and really late in the pipeline
and therefore other reducers aren't reapplied properly after redundant
phi/select elimination.

TEST=unittests
R=hpayer@chromium.org

Committed: https://crrev.com/ee98a1d76023e782750eaf9057b1520f0e6fb9d3
Cr-Commit-Position: refs/heads/master@{#25922}

[Hannes Payer (out of office), 2014-12-22 13:03:38]

lgtm

[commit-bot: I haz the power, 2014-12-22 13:06:50]

Patchset 1 (id:??) landed as
https://crrev.com/ee98a1d76023e782750eaf9057b1520f0e6fb9d3
Cr-Commit-Position: refs/heads/master@{#25922}

[Benedikt Meurer, 2014-12-22 13:06:52]

Committed patchset #1 (id:1) manually as
ee98a1d76023e782750eaf9057b1520f0e6fb9d3 (presubmit successful).