tree-optimization/126404 - fixup conversion detection in reductions

The following makes sure to check the pattern stmt against being a
conversion to skip but the original stmt for a mismatch in code.

	PR tree-optimization/126404
	* tree-vect-slp.cc (vect_analyze_slp_reduc_chain): Fixup
	conversion detection.

	* gcc.dg/vect/vect-pr126404.c: New testcase.