Replace uses of _mesa_bitcount with util_bitcount

Replace uses of _mesa_bitcount with util_bitcount

and _mesa_bitcount_64 with util_bitcount_64. This fixes a build problem
in nir for platforms that don't have popcount or popcountll, such as
32bit msvc.

v2: - Fix additional uses of _mesa_bitcount added after this was
      originally written

Acked-by: Eric Engestrom <eric.engestrom@intel.com> (v1)
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>