Fix VectorizeVectorScalarArithmetic statement insertion The traverser must avoid inserting two statements to the same position on a single traversal, so it doesn't trigger an assert. BUG=chromium:784078 TEST=angle_unittests Change-Id: I855054e62cc1b1cf4e6bb02af527954151c7d0e7 Reviewed-on: https://chromium-review.googlesource.com/771611 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>