rs6000: Use direct move for char/short vector CTOR [PR96933]

rs6000: Use direct move for char/short vector CTOR [PR96933]

This patch is to make vector CTOR with char/short leverage direct
move instructions when they are available.  With one constructed
test case, it can speed up 145% for char and 190% for short on P9.

Tested SPEC2017 x264_r at -Ofast on P9, it gets 1.61% speedup
(but based on unexpected SLP see PR96789).

Bootstrapped/regtested on powerpc64{,le}-linux-gnu P8 and
powerpc64le-linux-gnu P9.

gcc/ChangeLog:

	PR target/96933
	* config/rs6000/rs6000.c (rs6000_expand_vector_init): Use direct move
	instructions for vector construction with char/short types.
	* config/rs6000/rs6000.md (p8_mtvsrwz_v16qisi2): New define_insn.
	(p8_mtvsrd_v16qidi2): Likewise.

gcc/testsuite/ChangeLog:

	PR target/96933
	* gcc.target/powerpc/pr96933-1.c: New test.
	* gcc.target/powerpc/pr96933-2.c: New test.
	* gcc.target/powerpc/pr96933-3.c: New test.
	* gcc.target/powerpc/pr96933-4.c: New test.
	* gcc.target/powerpc/pr96933.h: New test.
	* gcc.target/powerpc/pr96933-run.h: New test.