Matrix compound multiplication was lacking transposes. Add them so that
results match normal multiplication.

BUG=angleproject:938
TEST=dEQP-GLES3.functional.shaders.matrix.mul_assign.*

Change-Id: I4c5159de0727afb4ee2831586f04fa817619de7a
Reviewed-on: https://chromium-review.googlesource.com/256366Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>