Unfolding the sequence operator ensures correct evaluation order when parts of
the expression need to get hoisted out of the expression to the surrounding
scope.

BUG=angleproject:1001
TEST=WebGL conformance tests

Change-Id: Ifd3093c9fb63d3e9842ebb4b9531b2f46875ad8a
Reviewed-on: https://chromium-review.googlesource.com/270660Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>