This fixes a bug where the re-creation of phi nodes leads to divergence. The fix makes sure that once a node created a phi node, it sticks to it and does not forget about it, even if the inputs suddenly agree again. The bug appeared on the trybots in https://codereview.chromium.org/2512733003/.

Also I added a line to mark effect phi nodes on the queue. This is unrelated, but seems to be an obvious ommission.

R=bmeurer@chromium.org
BUG=v8:5633

Review-Url: https://codereview.chromium.org/2522253002
Cr-Commit-Position: refs/heads/master@{#41239}