Add support for vectorising live-out values using SVE LASTB

Add support for vectorising live-out values using SVE LASTB

This patch uses the SVE LASTB instruction to optimise cases in which
a value produced by the final scalar iteration of a vectorised loop is
live outside the loop.  Previously this situation would stop us from
using a fully-masked loop.

2018-01-13  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* doc/md.texi (extract_last_@var{m}): Document.
	* optabs.def (extract_last_optab): New optab.
	* internal-fn.def (EXTRACT_LAST): New internal function.
	* internal-fn.c (cond_unary_direct): New macro.
	(expand_cond_unary_optab_fn): Likewise.
	(direct_cond_unary_optab_supported_p): Likewise.
	* tree-vect-loop.c (vectorizable_live_operation): Allow fully-masked
	loops using EXTRACT_LAST.
	* config/aarch64/aarch64-sve.md (aarch64_sve_lastb<mode>): Rename to...
	(extract_last_<mode>): ...this optab.
	(vec_extract<mode><Vel>): Update accordingly.

gcc/testsuite/
	* gcc.target/aarch64/sve/live_1.c: New test.
	* gcc.target/aarch64/sve/live_1_run.c: Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r256632