NxM matrices weren't mangled properly,
since only the nominal size was taken
into account, not the secondary size,
in the mangled name.

Change-Id: I94f4d8e1909cce085e8748bbe5355c5f17b0e7c2
Reviewed-on: https://swiftshader-review.googlesource.com/3271Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>