Use common helper functions instead of manually creating temporary
variable nodes. Also clean up the interface provided by the traverser.

BUG=angleproject:1597
Test=WebGL conformance tests

Change-Id: Ifd8d3815ff9e75e1a2040d65db9d4b3d6a9a9273
Reviewed-on: https://chromium-review.googlesource.com/403950Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>