tree-optimization/121830 - SLP cycle detection confused by nested cycle

tree-optimization/121830 - SLP cycle detection confused by nested cycle

The SLP reduc-index computation is confused by having an outer reduction
inner loop nested cycle fed by another non-reduction nested cycle.
Instead of undoing the unfortunate mixing of outer reduction inner
cycles with general nested cycles the following instead distinguishes
them by not setting STMT_VINFO_REDUC_DEF on the non-reduction nested
cycles.

	PR tree-optimization/121830
	* tree-vect-loop.cc (vect_analyze_scalar_cycles_1): Only
	set STMT_VINFO_REDUC_DEF on reductions.
	* tree-vect-slp.cc (vect_build_slp_tree_2): Identify reduction
	PHIs by a set STMT_VINFO_REDUC_DEF instead of their def type.

	* gcc.dg/vect/pr121830.c: New testcase.