Comment clarification for NULL => IV sv_setsv_flags optimization
More branch prediction hints for sv_setsv_flags

Dave's cachegrind benchmark says "small win":

          REF   THIS
       ------ ------
    Ir 105.35 105.91
    Dr 104.45 105.42
    Dw 105.42 105.17
  COND 104.33 104.58
   IND 107.04 106.76

COND_m 98.55  110.11
 IND_m 110.09 111.08