util: optimize bitcount on OpenCL

Fixes: bfc18b6fb1 ("libagx: drop libagx_popcount")
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Mary Guillemard <mary.guillemard@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34661>

src/util/bitscan.h

@@ -326,6 +326,8 @@ util_bitcount(unsigned n)
 {
 #if defined(HAVE___BUILTIN_POPCOUNT)
    return __builtin_popcount(n);
+#elif __OPENCL_VERSION__
+   return popcount(n);
 #else
    /* K&R classic bitcount.
     *
@@ -366,6 +368,8 @@ util_bitcount64(uint64_t n)
 {
 #ifdef HAVE___BUILTIN_POPCOUNTLL
    return __builtin_popcountll(n);
+#elif __OPENCL_VERSION__
+   return popcount(n);
 #else
    return util_bitcount((unsigned)n) + util_bitcount((unsigned)(n >> 32));
 #endif