Dump details of an attempt to register a jump threading path.

Dump details of an attempt to register a jump threading path.

The goal with these sets of patches is to improve the detailed dumps for
the threader, as I hope we eventually reach the point when I'm not
the only one looking at these dumps ;-).

This patch adds candidate paths to the detailed threading dumps to make it
easier to see the decisions the threader makes.  With it we can now
grep for the discovery logic in action:

$ grep ^path: a.ii.*thread*
a.ii.034t.ethread:path: 4->5->xx REJECTED
a.ii.034t.ethread:path: 3->5->8 SUCCESS
a.ii.034t.ethread:path: 4->5->6 SUCCESS
a.ii.034t.ethread:path: 0->2->xx REJECTED
a.ii.034t.ethread:path: 0->2->xx REJECTED
...
...
a.ii.111t.threadfull1:path: 14->22->23->xx REJECTED (unreachable)
a.ii.111t.threadfull1:path: 15->22->23->xx REJECTED (unreachable)
a.ii.111t.threadfull1:path: 16->22->23->xx REJECTED (unreachable)

In addition to this, if --param=threader-debug=all is used, one can see
the entire chain of events leading up to the ultimate threading
decision:

==============================================
path_range_query: compute_ranges for path: 2->5
 Registering killing_def (path_oracle) _3
 Registering killing_def (path_oracle) _1
range_defined_in_block (BB2) for _1 is _Bool VARYING
 Registering killing_def (path_oracle) _2
range_defined_in_block (BB2) for _2 is _Bool VARYING
range_defined_in_block (BB2) for _3 is _Bool VARYING
outgoing_edge_range_p for b_10(D) on edge 2->5 is int VARYING
...
... [BBs and gimple along path]
...
path: 2->5->xx REJECTED

Tested on x86-64 Linux.

gcc/ChangeLog:

	* tree-ssa-threadbackward.c
	(back_threader::maybe_register_path_dump): New.
	(back_threader::maybe_register_path): Call maybe_register_path_dump.