backwards threader cleanups

[Aldy Hernandez]

Hi.

Attached are misc cleanups to tree-ssa-threadbackwards.c and friends.
The main gist of the patch is making the path vectors live in the
heap, not GC.  But I also cleaned up some comments to reflect reality,
and renamed VAR_BB which could use a more meaningful name.  Finally, I
abstracted some common code to
register_jump_thread_path_if_profitable() in preparation for some
upcoming work by me :).

Tested on x86-64 Linux.

OK?
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