It checks that each phi label corresponds to an incoming edge, and that each incoming edge has a corresponding phi label.

It does not check that duplicate incoming edges get the same phi value.

Performance impact is minimal (~0.2%) despite the O(N^2) implementation.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4304
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1351433002 .