[turbofan] Move graph trimming functionality to dedicated GraphTrimmer.

[turbofan] Move graph trimming functionality to dedicated GraphTrimmer.

Up until now that was still mixed with control reduction in the
ControlReducer. This separation allows us to remove the horrible
Reducer::Finish hack and also do graph trimming at more appropriate
places in the pipeline (i.e. trim dead nodes after generic lowering,
which can also make nodes dead).

R=jarin@chromium.org,mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1188433010

Cr-Commit-Position: refs/heads/master@{#29077}