Implement support for assignments where the return value of the assignment
is not used in another part of the expression.

TEST=WebGL conformance tests
BUG=angleproject:960

Change-Id: Ibf9d71a75d27d139d2aabb5162ab04a0974321d3
Reviewed-on: https://chromium-review.googlesource.com/263222Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>