alias.c (alias_set_subset_of): Correctly handle asking if zero is a subset of an alias set with zero child.

2008-04-23  Richard Guenther  <rguenther@suse.de>

	* alias.c (alias_set_subset_of): Correctly handle asking
	if zero is a subset of an alias set with zero child.
	* tree-ssa-alias.c (have_common_aliases_p): Simplify logic.
	(compute_flow_insensitive_aliasing): Correctly walk all
	pointers.  Do not unnecessarily union sets.

From-SVN: r134597