Streamline vector lowering of VEC_COND_EXPR and vector comparisons

Streamline vector lowering of VEC_COND_EXPR and vector comparisons

The following makes sure to lower all VEC_COND_EXPRs that we cannot
trivially expand, likewise for comparisons.  In particular no longer
try to combine both in fancy ways.

	* tree-vect-generic.cc (expand_vector_comparison): Lower
	vector comparisons that we cannot trivially expand.  Remove
	code dealing with uses in VEC_COND_EXPRs.
	(expand_vector_condition): Lower vector conditions that we
	cannot trivially expand.  Remove code dealing with comparison
	mask definitions.
	(expand_vector_operation): Drop dce_ssa_names.
	(expand_vector_operations_1): Likewise.