Define vect_perm for variable-length SVE

Define vect_perm for variable-length SVE

Variable-length SVE now supports enough permutes to define vect_perm.

The change to vect_perm_supported is currently a no-op because the
function is only called with a count of 3.

2018-08-24  Richard Sandiford  <richard.sandiford@arm.com>

gcc/testsuite/
	* lib/target-supports.exp (vect_perm_supported): Only return
	false for variable-length vectors if the permute size is not
	a power of 2.
	(check_effective_target_vect_perm)
	(check_effective_target_vect_perm_byte)
	(check_effective_target_vect_perm_short): Remove check for
	variable-length vectors.
	* gcc.dg/vect/slp-23.c: Add an XFAIL for variable-length SVE.
	* gcc.dg/vect/slp-perm-10.c: Likewise.
	* gcc.dg/vect/slp-perm-9.c: Add an XFAIL for variable-length vectors.

From-SVN: r263834