This patch adds detection of average instructions:

       a = (((wide) b + (wide) c) >> 1);
   --> a = (wide) .AVG_FLOOR (b, c);

       a = (((wide) b + (wide) c + 1) >> 1);
   --> a = (wide) .AVG_CEIL (b, c);

in cases where users of "a" need only the low half of the result,
making the cast to (wide) redundant.  The heavy lifting was done by
earlier patches.

This showed up another problem in vectorizable_call: if the call is a
pattern definition statement rather than the main pattern statement,
the type of vectorised call might be different from the type of the
original statement.

2018-07-03  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/85694
	* doc/md.texi (avgM3_floor, uavgM3_floor, avgM3_ceil)
	(uavgM3_ceil): Document new optabs.
	* doc/sourcebuild.texi (vect_avg_qi): Document new target selector.
	* internal-fn.def (IFN_AVG_FLOOR, IFN_AVG_CEIL): New internal
	functions.
	* optabs.def (savg_floor_optab, uavg_floor_optab, savg_ceil_optab)
	(savg_ceil_optab): New optabs.
	* tree-vect-patterns.c (vect_recog_average_pattern): New function.
	(vect_vect_recog_func_ptrs): Add it.
	* tree-vect-stmts.c (vectorizable_call): Get the type of the zero
	constant directly from the associated lhs.

gcc/testsuite/
	PR tree-optimization/85694
	* lib/target-supports.exp (check_effective_target_vect_avg_qi): New
	proc.
	* gcc.dg/vect/vect-avg-1.c: New test.
	* gcc.dg/vect/vect-avg-2.c: Likewise.
	* gcc.dg/vect/vect-avg-3.c: Likewise.
	* gcc.dg/vect/vect-avg-4.c: Likewise.
	* gcc.dg/vect/vect-avg-5.c: Likewise.
	* gcc.dg/vect/vect-avg-6.c: Likewise.
	* gcc.dg/vect/vect-avg-7.c: Likewise.
	* gcc.dg/vect/vect-avg-8.c: Likewise.
	* gcc.dg/vect/vect-avg-9.c: Likewise.
	* gcc.dg/vect/vect-avg-10.c: Likewise.
	* gcc.dg/vect/vect-avg-11.c: Likewise.
	* gcc.dg/vect/vect-avg-12.c: Likewise.
	* gcc.dg/vect/vect-avg-13.c: Likewise.
	* gcc.dg/vect/vect-avg-14.c: Likewise.

From-SVN: r262335