The validation previously checked that two matrices in compound
multiplication are the exact same size, which isn't correct for
non-square matrices introduced in ESSL 3.00.

Instead, check that the matrix multiplication is valid and that the
resulting value has the same number of columns as the lvalue. The
number of rows in the result is taken from the lvalue so it doesn't
need to be checked.

BUG=angleproject:1431
TEST=angle_unittests

Change-Id: I6f32b7dc037d72c3c5cfdfffcda5d996e8450283
Reviewed-on: https://chromium-review.googlesource.com/356411Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>