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/431032Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>