This patch is the first part of the fix for PR 86725.  We would
treat x_1 in:

    outer1:
      x_1 = PHI <x_4(outer2), ...>;
      ...

    inner:
      x_2 = ...x_1...;
      ...
      x_3 = ...;
      ...

    outer2:
      x_4 = PHI <x_3(inner)>;
      ...

as a double reduction without checking what kind of statement x_2 is.
In practice it has to be a phi, since for other x_2, x_1 would simply
be a loop invariant that gets used for every inner loop iteration.

The idea with doing this patch first is that, by checking x_2 really
is a phi, we can hand off the validation of the rest of the reduction
to the phi analysis in the inner loop.

The test case is a variant of the one in the PR.

2018-08-22  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/86725
	* tree-vect-loop.c (vect_is_simple_reduction): When treating
	an outer loop phi as a double reduction, make sure that the
	single user of the phi result is an inner loop phi.

gcc/testsuite/
	PR tree-optimization/86725
	* gcc.dg/vect/no-scevccp-pr86725-1.c: New test.

From-SVN: r263773