Allow inner-loop reductions with variable-length vectors

Allow inner-loop reductions with variable-length vectors

While working on PR 86871, I noticed we were being overly restrictive
when handling variable-length vectors.  For:

  for (i : ...)
    {
      res = ...;
      for (j : ...)
        res op= ...;
      a[i] = res;
    }

we don't need a reduction operation (although we do for double
reductions like:

  res = ...;
  for (i : ...)
    for (j : ...)
      res op= ...;
  a[i] = res;

which must still be rejected).

2018-08-08  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (vectorizable_reduction): Allow inner-loop
	reductions for variable-length vectors.

gcc/testsuite/
	* gcc.target/aarch64/sve/reduc_8.c: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263451 138bc75d-0d04-0410-961f-82ee72b054a4