Reject compound assignments of the type scalar op= vector/matrix

scalar op vector/matrix produces a vector/matrix, which can't be
assigned to a scalar. Handle this correctly for addition, subtraction and
division. Multiplication was already handled correctly as a special case.

BUG=angle:832
TEST=WebGL conformance tests

Change-Id: I318cae8bf353a5c58c588876ce3f29d18389263c
Reviewed-on: https://chromium-review.googlesource.com/232601
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>