ifcombine: avoid unsound forwarder-enabled combinations [PR117723]

ifcombine: avoid unsound forwarder-enabled combinations [PR117723]

When ifcombining contiguous blocks, we can follow forwarder blocks and
reverse conditions to enable combinations, but when there are
intervening blocks, we have to constrain ourselves to paths to the
exit that share the PHI args with all intervening blocks.

Avoiding considering forwarders when intervening blocks were present
would match the preexisting test, but we can do better, recording in
case a forwarded path corresponds to the outer block's exit path, and
insisting on not combining through any other path but the one that was
verified as corresponding.  The latter is what this patch implements.

While at that, I've fixed some typos, introduced early testing before
computing the exit path to avoid it when computing it would be
wasteful, or when avoiding it can enable other sound combinations.

for  gcc/ChangeLog

	PR tree-optimization/117723
	* tree-ssa-ifcombine.cc (tree_ssa_ifcombine_bb): Record
	forwarder blocks in path to exit, and stick to them.  Avoid
	computing the exit if obviously not needed, and if that
	enables additional optimizations.
	(tree_ssa_ifcombine_bb_1): Fix typos.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/117723
	* gcc.dg/torture/ifcmb-1.c: New.