Merge BinaryOperatorReducer into the MachineOperatorReducer class.
It does not need `Revisit()` calls, because the newly inserted nodes are
visited anyway, and there are no other methods that need AdvancedReducer
there.

BUG=
R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1473073004

Cr-Commit-Position: refs/heads/master@{#32298}