ifcombine across noncontiguous blocks

ifcombine across noncontiguous blocks

Rework ifcombine to support merging conditions from noncontiguous
blocks.  This depends on earlier preparation changes.

The function that attempted to ifcombine a block with its immediate
predecessor, tree_ssa_ifcombine_bb, now loops over dominating blocks
eligible for ifcombine, attempting to combine with them.

The function that actually drives the combination of a pair of blocks,
tree_ssa_ifcombine_bb_1, now takes an additional parameter: the
successor of outer that leads to inner.

The function that recognizes if_then_else patterns is modified to
enable testing without distinguishing between then and else, or to
require nondegenerate conditions, that aren't worth combining with.

for  gcc/ChangeLog

	* tree-ssa-ifcombine.cc (recognize_if_then_else): Support
	relaxed then/else testing; require nondegenerate condition
	otherwise.
	(tree_ssa_ifcombine_bb_1): Add outer_succ_bb parm, use it
	instead of inner_cond_bb.  Adjust callers.
	(tree_ssa_ifcombine_bb): Loop over dominating outer blocks
	eligible for ifcombine.
	(pass_tree_ifcombine::execute): Noted potential need for
	changes to the post-combine logic.