Always qualify comma operator result with EvqTemporary in ESSL3, as
specified.

Also, it is possible that in the future some expressions are qualified
as EvqConst but they'd still have side effects, in which case discarding
them when they're the left operand of the comma operator would be wrong.
This would be the case if ANGLE allowed "(a = b).length()" for example.
For this reason it is better to check whether the left node has side
effects, rather than check its const qualification, and only discard it
if it doesn't.

Also, Intermediate::addComma() never returns null, so there's no need to
check the result.

BUG=angleproject:1201
TEST=WebGL conformance tests
     conformance/glsl/misc/sequence-operator-returns-constant.html
     conformance2/glsl3/sequence-operator-returns-non-constant.html

Change-Id: Ibfbd92baa4910b14c0dc8f8a3c3008440d191cd6
Reviewed-on: https://chromium-review.googlesource.com/311171Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>