Assert finished vectorizer pattern COND_EXPR transition

Assert finished vectorizer pattern COND_EXPR transition

The following places a few strathegic asserts so we do not end up
with COND_EXPRs with a comparison as the first operand during
vectorization.

	* tree-vect-slp.cc (vect_get_operand_map): Mark
	COMPARISON_CLASS_P COND_EXPR condition path unreachable.
	* tree-vect-stmts.cc (vect_is_simple_use): Likewise.
	(vectorizable_condition): Assert the COND_EXPR condition isn't
	COMPARISON_CLASS_P.