Reland "[turbofan] Reuse forward fixpoint algorithm in Typer by making it a Reducer."

This reverts commit 918ef9d712ab77bc33ffb8eb985a6fd5672602f6.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/780623002

Cr-Commit-Position: refs/heads/master@{#25646}