glsl: Use _mesa_bitcount to implement constant ir_unop_bit_count
glsl: Use _mesa_bitcount to implement constant ir_unop_bit_count

Signed-off-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler