The dead code elimination in SimplifiedLowering can eliminate pure nodes
if they don't have value uses. But some of those can indeed have control
inputs, i.e. Phi nodes do of course have a control input.

R=jarin@chromium.org
BUG=chromium:630923

Review-Url: https://codereview.chromium.org/2177133002
Cr-Commit-Position: refs/heads/master@{#37995}