Fix struct compound assignment being allowed in GLSL parsing The shader translator used to accept some invalid struct operations, like struct += struct and struct == struct with a different type. Fix this. BUG=angleproject:1476 TEST=angle_unittests Change-Id: Ia2303fc1f740da4d78242e094ee6004b07364973 Reviewed-on: https://chromium-review.googlesource.com/372718 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>