The trouble here is that the type of the induction variable might be
a bit ahead of the increment (JSAdd) operation's type. When we update
the type of the increment, we might only update the induction variable
type while the JSAdd type might be stale. If the induction variable typing
needs to fall back to normal phi typing (e.g., when the increment is not
an integer anymore), it might use the stale type.

To get around this, we fake monotonicity if we fallback to normal phi
typing. Another option would be to force re-typing of the increment
operation, but that seems to be harder to maintain.

BUG=chromium:644633

Review-Url: https://codereview.chromium.org/2320803002
Cr-Commit-Position: refs/heads/master@{#39261}