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>

src/intel/vulkan/anv_pipeline.c

@@ -364,7 +364,7 @@ populate_wm_prog_key(const struct gen_device_info *devinfo,
          key->color_outputs_valid |= (1 << i);
    }
 
-   key->nr_color_regions = _mesa_bitcount(key->color_outputs_valid);
+   key->nr_color_regions = util_bitcount(key->color_outputs_valid);
 
    key->replicate_alpha = key->nr_color_regions > 1 &&
                           ms_info && ms_info->alphaToCoverageEnable;