rs6000: Fix up easy_vector_constant_msb handling [PR101384]

rs6000: Fix up easy_vector_constant_msb handling [PR101384]

The following gcc.dg/pr101384.c testcase is miscompiled on
powerpc64le-linux.
easy_altivec_constant has code to try construct vector constants with
different element sizes, perhaps different from CONST_VECTOR's mode.  But as
written, that works fine for vspltis[bhw] cases, but not for the vspltisw
x,-1; vsl[bhw] x,x,x case, because that creates always a V16QImode, V8HImode
or V4SImode constant containing broadcasted constant with just the MSB set.
The vspltis_constant function etc. expects the vspltis[bhw] instructions
where the small [-16..15] or even [-32..30] constant is sign-extended to the
remaining step bytes, but that is not the case for the 0x80...00 constants,
with step > 1 we can't handle e.g.
{ 0x80, 0xff, 0xff, 0xff, 0x80, 0xff, 0xff, 0xff, 0x80, 0xff, 0xff, 0xff, 0x80, 0xff, 0xff, 0xff }
vectors but do want to handle e.g.
{ 0, 0, 0, 0x80, 0, 0, 0, 0x80, 0, 0, 0, 0x80, 0, 0, 0, 0x80 }
and similarly with copies > 1 we do want to handle e.g.
{ 0x80808080, 0x80808080, 0x80808080, 0x80808080 }.

2021-07-20  Jakub Jelinek  <jakub@redhat.com>

	PR target/101384
	* config/rs6000/rs6000-protos.h (easy_altivec_constant): Change return
	type from bool to int.
	* config/rs6000/rs6000.c (vspltis_constant): Fix up handling the
	EASY_VECTOR_MSB case if either step or copies is not 1.
	(vspltis_shifted): Fix comment typo.
	(easy_altivec_constant): Change return type from bool to int, instead
	of returning true return byte size of the element mode that should be
	used to synthetize the constant.
	* config/rs6000/predicates.md (easy_vector_constant_msb): Require
	that vspltis_shifted is 0, handle the case where easy_altivec_constant
	assumes using different vector mode from CONST_VECTOR's mode.
	* config/rs6000/altivec.md (easy_vector_constant_msb splitter): Use
	easy_altivec_constant to determine mode in which -1 >> -1 should be
	performed, use rs6000_expand_vector_init instead of gen_vec_initv4sisi.

	* gcc.dg/pr101384.c: New test.
	* gcc.target/powerpc/pr101384-1.c: New test.
	* gcc.target/powerpc/pr101384-2.c: New test.