PR tree-optimization/41783

	PR tree-optimization/41783
	* tree-ssa-alias.c (get_continuation_for_phi): Export, add a special
	case for simple diamonds
	* tree-ssa-alias.h (get_continuation_for_phi): Declare.
	* tree-ssa-pre.c (translate_vuse_through_block): Add same_valid
	argument, use alias oracle to skip some vdefs.
	(phi_translate_1): Change call to above, don't allocate new
	value ids if they can stay the same.
	(compute_avail): Allow vuse walking when looking up references.

testsuite/
	* gcc.dg/pr41783.c: New test.
	* gcc.dg/tree-ssa/ssa-pre-23.c: Adjust.
	* gcc.dg/tree-ssa/ssa-pre-24.c: Don't xfail anymore.
	* gcc.dg/tree-ssa/ssa-pre-27.c: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@153551 138bc75d-0d04-0410-961f-82ee72b054a4