The new implementation deals with cycles in the TF graph in two steps:
1) The lowering of phis is delayed to avoid cyclic dependencies.
2) The replacement nodes of phis are created already when the phi is
   pushed onto the stack so that other nodes can use these replacements
   for their lowering.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1844553002

Cr-Commit-Position: refs/heads/master@{#35126}