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>
