Now aggregate nodes are always built with their return type, op and
arguments set. They'll determine their qualifier and precision
automatically.

This fixes setting of gotPrecisionFromChildren in a few cases.

This will also make it easier to split TIntermAggregate further into
specialized classes if that is desired.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: I1fbe0c75679c517a22d44dfc1ea160ad7a7fdfda
Reviewed-on: https://chromium-review.googlesource.com/433468Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>