[turbofan] Add explicit set for nodes with weakened types.

[turbofan] Add explicit set for nodes with weakened types.

To ensure termination, we need to be sure that once we start weakening
a node, we keep weakening that node in subsequent re-typings. Until now,
we were guessing that we previously weakened from the type. This
change introduces a set of nodes that have already be weakened, so
that we have a reliable way to detect previous weakening.

BUG=chromium:468799
LOG=n
R=bmeurer@chromium.org

Committed: https://crrev.com/adb8867337b75282f9e61251d015b77d4f92ad6b
Cr-Commit-Position: refs/heads/master@{#27331}

[Jarin, 2015-03-19 16:35:55]

Could you take a look, please?

[Benedikt Meurer, 2015-03-19 17:08:36]

lgtm

[Jarin, 2015-03-20 09:10:43]

The CQ bit was checked by jarin@chromium.org

[commit-bot: I haz the power, 2015-03-20 09:10:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1019883002/1

[commit-bot: I haz the power, 2015-03-20 09:12:48]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-20 09:13:12]

Patchset 1 (id:??) landed as
https://crrev.com/adb8867337b75282f9e61251d015b77d4f92ad6b
Cr-Commit-Position: refs/heads/master@{#27331}