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 }.

This is a simpler version of the fix for backports, which limits the EASY_VECTOR_MSB case
matching to step == 1 && copies == 1, because that is the only case the
splitter handles correctly.

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

	PR target/101384
	* config/rs6000/rs6000.c (vspltis_constant): Accept EASY_VECTOR_MSB
	only if step and copies are equal to 1.

	* gcc.dg/pr101384.c: New test.

(cherry picked from commit dc386b020869ad0095cf58f8c76a40ea457e7a2c)