Fix validating non-square matrix compound multiplication 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/356411 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>