Generate XXSPLTIW on power10.

This patch adds support to automatically generate the ISA 3.1 XXSPLTIW
instruction for V8HImode, V4SImode, and V4SFmode vectors.  It does this by
adding support for vector constants that can be used, and adding a
VEC_DUPLICATE pattern to generate the actual XXSPLTIW instruction.

Add the eP constraint to recognize constants that can be loaded into
vector registers with a single prefixed instruction such as xxspltiw and
xxspltidp.

I added 4 new tests to test loading up V16QI, V8HI, V4SI, and V4SF vector
constants.

2021-12-14  Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/constraints.md (eP): Update comment.
	* config/rs6000/predicates.md (easy_fp_constant): Add support for
	generating XXSPLTIW.
	(vsx_prefixed_constant): New predicate.
	(easy_vector_constant): Add support for
	generating XXSPLTIW.
	* config/rs6000/rs6000-protos.h (prefixed_xxsplti_p): New
	declaration.
	(constant_generates_xxspltiw): Likewise.
	* config/rs6000/rs6000.c (xxspltib_constant_p): Generate XXSPLTIW
	if possible instead of XXSPLTIB and sign extending the constant.
	(output_vec_const_move): Add support for XXSPLTIW.
	(prefixed_xxsplti_p): New function.
	(constant_generates_xxspltiw): New function.
	* config/rs6000/rs6000.md (prefixed attribute): Add support to
	mark XXSPLTI* instructions as being prefixed.
	* config/rs6000/rs6000.opt (-msplat-word-constant): New debug
	switch.
	* config/rs6000/vsx.md (vsx_mov<mode>_64bit): Add support for
	generating XXSPLTIW or XXSPLTIDP.
	(vsx_mov<mode>_32bit): Likewise.
	* doc/md.texi (PowerPC and IBM RS6000 constraints): Document the
	eP constraint.

gcc/testsuite/

	* gcc.target/powerpc/vec-splat-constant-v16qi.c: New test.
	* gcc.target/powerpc/vec-splat-constant-v4sf.c: New test.
	* gcc.target/powerpc/vec-splat-constant-v4si.c: New test.
	* gcc.target/powerpc/vec-splat-constant-v8hi.c: New test.
	* gcc.target/powerpc/vec-splati-runnable.c: Update insn count.