Previously, the type of the return value of all function calls was set to
the type of the return value in the function signature. This did not
carry precision information.

This patch changes this so that the return value precision is set
correctly for built-in functions. For single-argument math functions, it
mostly depends on that addUnaryMath sets the type of the return value to
be the same as the type of the operand. The type is replaced but the
precision information from the operand type is retained when needed. For
multi-argument math functions, precision is determined based on all the
nodes in the aggregate after the type has been set. For texture
functions, the precision is set according the sampler type as per ESSL
1.0 spec. For textureSize, the precision is always highp as per ESSL 3.0
spec.

BUG=angle:787

Change-Id: I48448e3ffe38656b91177dee9b60dd07a03cd095
Reviewed-on: https://chromium-review.googlesource.com/224951Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>