Restrict bb-slp-40.c to targets with VnQI addition (PR 92366)

Restrict bb-slp-40.c to targets with VnQI addition (PR 92366)

bb-slp-40.c fails on SPARC targets without VIS4 because it
requires addition on vectors of bytes.  There doesn't seem to be
an existing target selector for this, so I added vect_char_add.
(Wasn't sure whether to use vect_char_add, for consistency
with vect_no_int_add/vect_int_mult etc., or vect_add_char for
consistency with vect_shift_char etc.)

I took the target list from vect_int and removed targets that didn't
seem to support the operation (namely sparc*, since we don't seem to
have any test for VIS4, niagara7 or m8, and alpha*-*-*.)

2019-11-20  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR testsuite/92366
	* doc/sourcebuild.texi (vect_char_add): Document.

gcc/testsuite/
	PR testsuite/92366
	* lib/target-supports.exp (check_effective_target_vect_char_add):
	New proc.
	* gcc.dg/vect/bb-slp-40.c: Require vect_char_add instead of vect_int.

From-SVN: r278532