Use GetOperatorString in intermediate output The special names for built-ins used in intermediate output are kept only where they add some significant value, like clearly disambiguating between component-wise and non-component-wise ops. Otherwise intermediate output now relies on GetOperatorString to give names for ops. This will make it easier to add new built-in functions. This also fixes intermediate output for TIntermSwizzle, dFdx, dFdy and fwidth. BUG=angleproject:1730 TEST=angle_unittests Change-Id: Ifa06b886ad8e24b0820513adae8decd60d80a0e7 Reviewed-on: https://chromium-review.googlesource.com/431032 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>