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

Review URL: https://codereview.chromium.org/1019883002

Cr-Commit-Position: refs/heads/master@{#27331}