Do not use out of range visitor ids.

BUG=

Review URL: https://codereview.chromium.org/822693002

Cr-Commit-Position: refs/heads/master@{#25920}
Work-around to make webkit test suite work with optdebug==2.

The test driver doesn't know at the moment if optdebug 1 or 2 has been used (the first is used on the bots, the second
locally in quickcheck). It can't know therefore if slow
asserts can be enabled or not. Trying to enable them leads
to a warning if they weren't compiled, which causes webkit
1:1 text comparison to fail.

Review URL: https://codereview.chromium.org/819033002

Cr-Commit-Position: refs/heads/master@{#25921}
[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}