This CL implements loop assignment analysis, a pass over a loop's body
to record local variables that are assigned. This pre-pass is similar
to that done on the JavaScript AST for the same reason: avoid introducing
too many phis at loop headers when building a graph.

R=bradnelson@chromium.org,ahaas@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1617723003

Cr-Commit-Position: refs/heads/master@{#33486}