Clean up TIntermAggregate folding The division between 2- and 3-parameter built-ins is superfluous and can be removed. Also folding support for ESSL 3.00 is complete, so the comments about needing to add folding support for more functions can be removed. BUG=angleproject:1730 TEST=angle_unittests Change-Id: I5ca93a957a98a7740eca506e44af48e7b4b83bad Reviewed-on: https://chromium-review.googlesource.com/431108 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>