Add a vect_unaligned_possible target selector

Add a vect_unaligned_possible target selector

This patch adds a target selector that says whether we can ever
generate an "unaligned" accesses, where "unaligned" is relative
to the target's preferred vector alignment.  This is already true if:

   vect_no_align && { ! vect_hw_misalign }

i.e. if the target doesn't have any alignment mechanism and also
doesn't allow unaligned accesses.  It is also true (for the things
tested by gcc.dg/vect) if the target only wants things to be aligned
to an element; in that case every normal scalar access is "vector aligned".

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_unaligned_possible): Document.

gcc/testsuite/
	* lib/target-supports.exp
	(check_effective_target_vect_unaligned_possible): New proc.
	* gcc.dg/vect/slp-25.c: Extend XFAIL of peeling for alignment from
	vect_no_align && { ! vect_hw_misalign } to ! vect_unaligned_possible.
	* gcc.dg/vect/vect-multitypes-1.c: Likewise.
	* gcc.dg/vect/vect-109.c: XFAIL vectorisation of an unaligned
	access to ! vect_unaligned_possible.
	* gcc.dg/vect/vect-33.c: Likewise.
	* gcc.dg/vect/vect-42.c: Likewise.
	* gcc.dg/vect/vect-56.c: Likewise.
	* gcc.dg/vect/vect-60.c: Likewise.
	* gcc.dg/vect/vect-96.c: Likewise.
	* gcc.dg/vect/vect-peel-1.c: Likewise.
	* gcc.dg/vect/vect-27.c: Extend XFAIL of unaligned vectorization from
	vect_no_align && { ! vect_hw_misalign } to ! vect_unaligned_possible.
	* gcc.dg/vect/vect-29.c: Likewise.
	* gcc.dg/vect/vect-44.c: Likewise.
	* gcc.dg/vect/vect-48.c: Likewise.
	* gcc.dg/vect/vect-50.c: Likewise.
	* gcc.dg/vect/vect-52.c: Likewise.
	* gcc.dg/vect/vect-72.c: Likewise.
	* gcc.dg/vect/vect-75-big-array.c: Likewise.
	* gcc.dg/vect/vect-75.c: Likewise.
	* gcc.dg/vect/vect-77-alignchecks.c: Likewise.
	* gcc.dg/vect/vect-77-global.c: Likewise.
	* gcc.dg/vect/vect-78-alignchecks.c: Likewise.
	* gcc.dg/vect/vect-78-global.c: Likewise.
	* gcc.dg/vect/vect-multitypes-3.c: Likewise.
	* gcc.dg/vect/vect-multitypes-4.c: Likewise.
	* gcc.dg/vect/vect-multitypes-6.c: Likewise.
	* gcc.dg/vect/vect-peel-4.c: Likewise.
	* gcc.dg/vect/vect-peel-3.c: Likewise, and also for peeling
	for alignment.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r254594