intel/fs: use uint type for per_slot_offset at GS

This helps us to compact original instruction:

mul(8)  g3<1>D  g6<8,8,1>UD  0x00000006UD { align1 1Q };

So now we emit:

mul(8)  g3<1>UD g6<8,8,1>UD  0x00000006UD { align1 1Q compacted };

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>

src/intel/compiler/brw_fs_visitor.cpp

@@ -602,7 +602,7 @@ fs_visitor::emit_urb_writes(const fs_reg &gs_vertex_count)
          per_slot_offsets = brw_imm_ud(output_vertex_size_owords *
                                        gs_vertex_count.ud);
       } else {
-         per_slot_offsets = vgrf(glsl_type::int_type);
+         per_slot_offsets = vgrf(glsl_type::uint_type);
          bld.MUL(per_slot_offsets, gs_vertex_count,
                  brw_imm_ud(output_vertex_size_owords));
       }