tree-optimization/106514 - add --param max-jump-thread-paths

The following adds a limit for the exponential greedy search of
the backwards jump threader.  The idea is to limit the search
space in a way that the paths considered are the same if the search
were in BFS order rather than DFS.  In particular it stops considering
incoming edges into a block if the product of the in-degrees of
blocks on the path exceeds the specified limit.

When considering the low stmt copying limit of 7 (or 1 in the size
optimize case) this means the degenerate case with maximum search
space is a sequence of conditions with no actual code

  B1
   |\
   | empty
   |/
  B2
   |\
   ...
  Bn
   |\

GIMPLE_CONDs are costed 2, an equivalent GIMPLE_SWITCH already 4, so
we reach 7 already with 3 middle conditions (B1 and Bn do not count).
The search space would be 2^4 == 16 to reach this.  The FSM threads
historically allowed for a thread length of 10 but is really looking
for a single multiway branch threaded across the backedge.  I've
chosen the default of the new parameter to 64 which effectively
limits the outdegree of the switch statement (the cases reaching the
backedge) to that number (divided by 2 until I add some special
pruning for FSM threads due to the loop header indegree).  The
testcase ssa-dom-thread-7.c requires 56 at the moment (as said,
some special FSM thread pruning of considered edges would bring
it down to half of that), but we now get one more threading
and quite some more in later threadfull.  This testcase seems to
be difficult to check for expected transforms.

The new testcases add the degenerate case we currently thread
(without deciding whether that's a good idea ...) plus one with
an approripate limit that should prevent the threading.

This obsoletes the mentioned --param max-fsm-thread-length but
I am not removing it as part of this patch.  When the search
space is limited the thread stmt size limit effectively provides
max-fsm-thread-length.

The param with its default does not help PR106514 enough to unleash
path searching with the higher FSM stmt count limit.

	PR tree-optimization/106514
	* params.opt (max-jump-thread-paths): New.
	* doc/invoke.texi (max-jump-thread-paths): Document.
	* tree-ssa-threadbackward.cc (back_threader::find_paths_to_names):
	Honor max-jump-thread-paths, take overall_path argument.
	(back_threader::find_paths): Pass 1 as initial overall_path.

	* gcc.dg/tree-ssa/ssa-thread-16.c: New testcase.
	* gcc.dg/tree-ssa/ssa-thread-17.c: Likewise.
	* gcc.dg/tree-ssa/ssa-dom-thread-7.c: Adjust.