Unfold sequence operator when operations inside need unfolding 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/270660 Tested-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>