The opcodes

  push %seg
  pop %seg
  l%seg, %mem, %reg  (e.g. lds/les/lss/lfs/lgs)

all have an segment register encoded in the instruction.  To allow reuse,
decode the segment number into src2 during the decode stage instead of the
execution stage.
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>