Add vect_perm3_* target selectors

Add vect_perm3_* target selectors

SLP load permutation fails if any individual permutation requires more
than two vector inputs.  For 128-bit vectors, it's possible to permute
3 contiguous loads of 32-bit and 8-bit elements, but not 16-bit elements
or 64-bit elements.  The results are reversed for 256-bit vectors,
and so on for wider vectors.

This patch adds a routine that tests whether a permute will require
three vectors for a given vector count and element size, then adds
vect_perm3_* target selectors for the cases that we currently use.

2017-11-09  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* doc/sourcebuild.texi (vect_perm_short, vect_perm_byte): Document
	previously undocumented selectors.
	(vect_perm3_byte, vect_perm3_short, vect_perm3_int): Document.

gcc/testsuite/
	* lib/target-supports.exp (vect_perm_supported): New proc.
	(check_effective_target_vect_perm3_int): Likewise.
	(check_effective_target_vect_perm3_short): Likewise.
	(check_effective_target_vect_perm3_byte): Likewise.
	* gcc.dg/vect/slp-perm-1.c: Expect SLP load permutation to
	succeed if vect_perm3_int.
	* gcc.dg/vect/slp-perm-5.c: Likewise.
	* gcc.dg/vect/slp-perm-6.c: Likewise.
	* gcc.dg/vect/slp-perm-7.c: Likewise.
	* gcc.dg/vect/slp-perm-8.c: Likewise vect_perm3_byte.
	* gcc.dg/vect/slp-perm-9.c: Likewise vect_perm3_short.
	Use vect_perm_short instead of vect_perm.  Add a scan-tree-dump-not
	test for vect_perm3_short targets.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@254592 138bc75d-0d04-0410-961f-82ee72b054a4