i965/fs: Return result of image atomic in a register of the expected type.

So the result is of float type if we're implementing the float
overload of imageAtomicExchange.  This is the only back-end change
required to support OES_shader_image_atomic AFAICT.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/mesa/drivers/dri/i965/brw_fs_surface_builder.cpp

@@ -1125,7 +1125,7 @@ namespace brw {
          if (rsize && pred)
             set_predicate(pred, bld.SEL(tmp, tmp, brw_imm_d(0)));
 
-         return tmp;
+         return retype(tmp, src0.type);
       }
    }
 }