intel/fs/swsb: SCHEDULING_FENCE only emits SYNC_NOP

It's not really unordered in the sense that it can still stall on
ordered things and we don't need a SYNC_NOP for that because it is a
SYNC_NOP.  However, it also doesn't count when computing instruction
distances.

Fixes: 18e72ee210 "intel/fs: Add FS_OPCODE_SCHEDULING_FENCE"
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6781>

src/intel/compiler/brw_fs_scoreboard.cpp

@@ -77,6 +77,7 @@ namespace {
       case BRW_OPCODE_DO:
       case SHADER_OPCODE_UNDEF:
       case FS_OPCODE_PLACEHOLDER_HALT:
+      case FS_OPCODE_SCHEDULING_FENCE:
          return 0;
       default:
          /* Note that the following is inaccurate for virtual instructions