[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

Review URL: https://codereview.chromium.org/817243003

Cr-Commit-Position: refs/heads/master@{#25922}
Use weak cell in monomorphic KeyedStore IC.

BUG=v8:3629
LOG=N

Review URL: https://codereview.chromium.org/817873003

Cr-Commit-Position: refs/heads/master@{#25923}