This patch adds support to vectorize sum of abslolute differences (SAD_EXPR)

This patch adds support to vectorize sum of abslolute differences (SAD_EXPR)
using SVE.

Given this input code:

int
sum_abs (uint8_t *restrict x, uint8_t *restrict y, int n)
{
  int sum = 0;

  for (int i = 0; i < n; i++)
    {
      sum += __builtin_abs (x[i] - y[i]);
    }

  return sum;
}

The resulting SVE code is:

0000000000000000 <sum_abs>:
   0:	7100005f 	cmp	w2, #0x0
   4:	5400026d 	b.le	50 <sum_abs+0x50>
   8:	d2800003 	mov	x3, #0x0                   	// #0
   c:	93407c42 	sxtw	x2, w2
  10:	2538c002 	mov	z2.b, #0
  14:	25221fe0 	whilelo	p0.b, xzr, x2
  18:	2538c023 	mov	z3.b, #1
  1c:	2518e3e1 	ptrue	p1.b
  20:	a4034000 	ld1b	{z0.b}, p0/z, [x0, x3]
  24:	a4034021 	ld1b	{z1.b}, p0/z, [x1, x3]
  28:	0430e3e3 	incb	x3
  2c:	0520c021 	sel	z1.b, p0, z1.b, z0.b
  30:	25221c60 	whilelo	p0.b, x3, x2
  34:	040d0420 	uabd	z0.b, p1/m, z0.b, z1.b
  38:	44830402 	udot	z2.s, z0.b, z3.b
  3c:	54ffff21 	b.ne	20 <sum_abs+0x20>  // b.any
  40:	2598e3e0 	ptrue	p0.s
  44:	04812042 	uaddv	d2, p0, z2.s
  48:	1e260040 	fmov	w0, s2
  4c:	d65f03c0 	ret
  50:	1e2703e2 	fmov	s2, wzr
  54:	1e260040 	fmov	w0, s2
  58:	d65f03c0 	ret

Notice how udot is used inside a fully masked loop.

gcc/Changelog:

2019-05-07  Alejandro Martinez  <alejandro.martinezvicente@arm.com>

	* config/aarch64/aarch64-sve.md (<su>abd<mode>_3): New define_expand.
	(aarch64_<su>abd<mode>_3): Likewise.
	(*aarch64_<su>abd<mode>_3): New define_insn.
	(<sur>sad<vsi2qi>): New define_expand.
	* config/aarch64/iterators.md: Added MAX_OPP attribute.
	* tree-vect-loop.c (use_mask_by_cond_expr_p): Add SAD_EXPR.
	(build_vect_cond_expr): Likewise.

gcc/testsuite/Changelog:

2019-05-07  Alejandro Martinez  <alejandro.martinezvicente@arm.com>

	* gcc.target/aarch64/sve/sad_1.c: New test for sum of absolute
	differences.

From-SVN: r270975