Setting the type for folded aggregate nodes should work in a similar way
as other constant folding. Common functionality between the different
folding functions is refactored into a single function.

TEST=dEQP-GLES3.functional.shaders.constant_expressions.*
BUG=angleproject:817

Change-Id: Ie0be561f4a30e52e52d570ff0b2bdb426f6e4f7a
Reviewed-on: https://chromium-review.googlesource.com/275186Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>