Use all the vector elements correctly when constant folding non-square
outerProduct. Previously the code used to discard some elements of the
smaller outerProduct operand. Also clear up confusion about matrix
rows/columns in matrix constant folding code in general.

BUG=angleproject:1482
TEST=angle_unittests

Change-Id: I7cba8f97a92b875de01e57255d54258cdfd92a47
Reviewed-on: https://chromium-review.googlesource.com/377298Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>