[PATCH, rs6000] int128 sign extention instructions (partial prereq)

[PATCH, rs6000] int128 sign extention instructions (partial prereq)

Hi
  This is a sub-set of the 128-bit sign extension support patch series
  that will be fully implemented in a subsequent patch from Carl.
  This is a necessary pre-requisite for the vector-load/store rightmost
  element patch that follows in this thread.

  [v2] Refreshed and touched up per review comments.
  - updated set_attr entries.  removed superfluous set_attr entries.
  - moved define_insn and define_expand entries to vsx.md.

gcc/ChangeLog:
	* config/rs6000/vsx.md (enum unspec): Add
	UNSPEC_EXTENDDITI2 and UNSPEC_MTVSRD_DITI_W1 entries.
	(mtvsrdd_diti_w1, extendditi2_vector): New define_insns.
	(extendditi2): New define_expand.