GetLocationCount() returned secondary size for fields of a struct.  For
matrices however, the correct value would be the primary size.

This is a similar fix to 6423b7fc.

Bug: angleproject:4200
Change-Id: I2c69b9454729993010766fcde0cabec986b7429d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2021738Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>