rs6000: Splat vector small V2DI constants with vspltisw and vupkhsw

rs6000: Splat vector small V2DI constants with vspltisw and vupkhsw

This patch adds a new insn for vector splat with small V2DI constants on P8.
If the value of constant is in RANGE (-16, 15) but not 0 or -1, it can be
loaded with vspltisw and vupkhsw on P8.

gcc/
	PR target/104124
	* config/rs6000/altivec.md (*altivec_vupkhs<VU_char>_direct): Rename
	to...
	(altivec_vupkhs<VU_char>_direct): ...this.
	* config/rs6000/predicates.md (vspltisw_vupkhsw_constant_split): New
	predicate to test if a constant can be loaded with vspltisw and
	vupkhsw.
	(easy_vector_constant): Call vspltisw_vupkhsw_constant_p to Check if
	a vector constant can be synthesized with a vspltisw and a vupkhsw.
	* config/rs6000/rs6000-protos.h (vspltisw_vupkhsw_constant_p):
	Declare.
	* config/rs6000/rs6000.cc (vspltisw_vupkhsw_constant_p): New
	function to return true if OP mode is V2DI and can be synthesized
	with vupkhsw and vspltisw.
	* config/rs6000/vsx.md (*vspltisw_v2di_split): New insn to load up
	constants with vspltisw and vupkhsw.

gcc/testsuite/
	PR target/104124
	* gcc.target/powerpc/pr104124.c: New.