mesa/st: Move the dword slot counting function to glsl_types as well.

mesa/st: Move the dword slot counting function to glsl_types as well.

To implement NIR-to-TGSI, we need to be able to get the size of the
uniform variable for the TGSI declaration, not just the
.driver_location.  With its location in mesa/st, drivers couldn't link
to it from nir-to-tgsi.

This feels like a common enough function to want, so let's share it in
the core compiler.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3297>