GetLocationCount() is currently returning the number of rows in the
matrix, rather than the columns, which leads to shader validation
errors (and test failures).   This fix returns the number of columns
in a matrix.

Bug: angleproject:4200
Test: dEQP-GLES31.functional.separate_shader.random.63
Change-Id: I8d25eb3733c2ddbe53ff54794f480c1d43e22a88
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1952173Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>