tree-optimization/109025 - fixup double reduction detection

The following closes a gap in double reduction detection where we
in the outer loop analysis fail to verify the inner LC PHI use is
the latch definition of the inner loop PHI.  That latch definition
is used to detect that an inner loop is part of a double reduction
when later doing the inner loop analysis.

	PR tree-optimization/109025
	* tree-vect-loop.cc (vect_is_simple_reduction): Verify
	the inner LC PHI use is the inner loop PHI latch definition
	before classifying an outer PHI as double reduction.

	* gcc.dg/vect/pr109025.c: New testcase.