[turbofan] Handle collisions properly in value numbering.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/795433003

Cr-Commit-Position: refs/heads/master@{#25857}
Allow timeout on regexp test

R=marja@chromium.org
BUG=

Review URL: https://codereview.chromium.org/798713004

Cr-Commit-Position: refs/heads/master@{#25858}