The goal of the Int64Reducer is to replace all int64 nodes in a tf graph
with a set of int32 nodes such that 64 bit tf functions can be executed
on 32 bit platforms. At the moment the Int64Reducer only replaces
Int64Constants, TruncateInt64ToInt32, and Word64And.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1655883002

Cr-Commit-Position: refs/heads/master@{#33721}