[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

Committed: https://crrev.com/80a6e53935a8205f0b5032759ecdf8aecb8d2a38
Cr-Commit-Position: refs/heads/master@{#29077}

[Benedikt Meurer, 2015-06-17 09:30:05]

Hey Michi, Jaro,

This separates the control reduction from the dead node trimming. There's no
change in functionality. Please take a look.

Thanks,
Benedikt

[Jarin, 2015-06-17 09:41:03]

lgtm. very nice.

[Michael Starzinger, 2015-06-17 10:32:48]

LGTM.

[Benedikt Meurer, 2015-06-17 10:33:01]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-06-17 10:33:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1188433010/1

[commit-bot: I haz the power, 2015-06-17 10:56:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-17 10:56:48]

Patchset 1 (id:??) landed as
https://crrev.com/80a6e53935a8205f0b5032759ecdf8aecb8d2a38
Cr-Commit-Position: refs/heads/master@{#29077}