[Aarch64][SVE] Dot product support

[Aarch64][SVE] Dot product support

This patch does two things. For the general vectoriser, it adds support to
perform fully masked reductions over expressions that don't support masking.
This is achieved by using VEC_COND_EXPR where possible.  At the moment this is
implemented for DOT_PROD_EXPR only, but the framework is there to extend it to
other expressions.

Related to that, this patch adds support to vectorize dot product using SVE.  It
also uses the new functionality to ensure that the resulting loop is masked.

Given this input code:

uint32_t
dotprod (uint8_t *restrict x, uint8_t *restrict y, int n)
{
  uint32_t sum = 0;

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

  return sum;
}

The resulting SVE code is:

0000000000000000 <dotprod>:
   0:	7100005f 	cmp	w2, #0x0
   4:	5400024d 	b.le	4c <dotprod+0x4c>
   8:	d2800003 	mov	x3, #0x0                   	// #0
   c:	93407c42 	sxtw	x2, w2
  10:	2538c001 	mov	z1.b, #0
  14:	25221fe0 	whilelo	p0.b, xzr, x2
  18:	2538c003 	mov	z3.b, #0
  1c:	d503201f 	nop
  20:	a4034002 	ld1b	{z2.b}, p0/z, [x0, x3]
  24:	a4034020 	ld1b	{z0.b}, p0/z, [x1, x3]
  28:	0430e3e3 	incb	x3
  2c:	0523c000 	sel	z0.b, p0, z0.b, z3.b
  30:	25221c60 	whilelo	p0.b, x3, x2
  34:	44820401 	udot	z1.s, z0.b, z2.b
  38:	54ffff41 	b.ne	20 <dotprod+0x20>  // b.any
  3c:	2598e3e0 	ptrue	p0.s
  40:	04812021 	uaddv	d1, p0, z1.s
  44:	1e260020 	fmov	w0, s1
  48:	d65f03c0 	ret
  4c:	1e2703e1 	fmov	s1, wzr
  50:	1e260020 	fmov	w0, s1
  54:	d65f03c0 	ret

Notice how udot is used inside a fully masked loop.

I tested this patch in an aarch64 machine bootstrapping the compiler and
running the checks.
gcc/Changelog:

2019-05-02  Alejandro Martinez  <alejandro.martinezvicente@arm.com>

	* config/aarch64/aarch64-sve.md (<sur>dot_prod<vsi2qi>): Taken from SVE
	ACLE branch.
	* config/aarch64/iterators.md: Copied Vetype_fourth, VSI2QI and vsi2qi from
	SVE ACLE branch.
	* tree-vect-loop.c (use_mask_by_cond_expr_p): New function to check if a
	VEC_COND_EXPR be inserted to emulate a conditional internal function.
	(build_vect_cond_expr): Emit the VEC_COND_EXPR.
	(vectorizable_reduction): Use the functions above to vectorize in a
	fully masked loop codes that don't have a conditional internal
	function.

gcc/testsuite/Changelog:

2019-05-02  Alejandro Martinez  <alejandro.martinezvicente@arm.com>

	* gcc.target/aarch64/sve/dot_1.c: New test for dot product.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@270790 138bc75d-0d04-0410-961f-82ee72b054a4