BinopMatcher does not notify the reducers using it when it flips inputs to commutative operators. This leads to value numbering not being re-executed in this case. Together with the fact that value numbering might still reduce such a modified node in the case of a hash collision merging the buckets of two equivalent nodes, this leads to unpredictable behaviour.

This is the easiest fix for the problem: Always running value numbering last. This is also a performance improvement because value numbering never changes but only replaces nodes.

R=mstarzinger@chromium.org

Review-Url: https://codereview.chromium.org/2728983002
Cr-Commit-Position: refs/heads/master@{#43552}