This will immediately remove dead code from the graph once any of
the advanced reducers inserts it. Also changes the GraphReducer to
use the canonical Dead node for ReplaceWithValue.

R=jarin@chromium.org

Committed: https://crrev.com/88a40c5fb381924b1c0b2403dc582bceb2abe5da
Cr-Commit-Position: refs/heads/master@{#29217}

Review URL: https://codereview.chromium.org/1206533002.

Cr-Commit-Position: refs/heads/master@{#29225}