Comma nodes always have just two parameters. If there's an expression
with several commas in the middle, it's parsed as a tree of comma
operations. It makes more sense to represent it as a binary node
rather than an aggregate node.

After this patch, TIntermAggregate is still used for function
prototypes, function parameter lists, function calls, and variable and
invariant declarations.

BUG=angleproject:1490
TEST=angle_unittests, angle_end2end_tests

Change-Id: I66be10624bf27bcf25987b4d93958d4a07600771
Reviewed-on: https://chromium-review.googlesource.com/397320Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>