tree-optimization/109219 - avoid looking at STMT_SLP_TYPE

The following avoids looking at STMT_SLP_TYPE apart from the only
place needing it - transform and analysis of non-SLP loop stmts.
In particular it doesn't have a reliable meaning on SLP representatives
which are also passed as stmt_vinfo to vectorizable_* routines.  The
proper way to check in those is to look for the slp_node argument
instead.

	PR tree-optimization/109219
	* tree-vect-loop.cc (vectorizable_reduction): Check
	slp_node, not STMT_SLP_TYPE.
	* tree-vect-stmts.cc (vectorizable_condition): Likewise.
	* tree-vect-slp.cc (vect_slp_analyze_node_operations_1):
	Remove assertion on STMT_SLP_TYPE.

	* gcc.dg/torture/pr109219.c: New testcase.