rs6000: Rework vsx_extract_<mode>

rs6000: Rework vsx_extract_<mode>

Extracting the left and right halfs of a vector are entirely different
operations.  Things are simpler if they are separate define_insns, and
it is easy to get rid of the "wD" constraint use then.

This also give the variant that is a no-op copy its own alternative, of
length 0 (and this, cost 0, making it more likely RA will choose it.

2022-10-05  Segher Boessenkool  <segher@kernel.crashing.org>

	* config/rs6000/vsx.md (vsx_extract_<mode>): Replace define_insn by a
	define_expand.  Split the contents to...
	(*vsx_extract_<mode>_0): ... this.  Rewrite.
	(*vsx_extract_<mode>_1): ... and this.  Rewrite.