Add debug counters to back threader.

Chasing down stage3 miscomparisons is never fun, and having no way to
distinguish between jump threads registered by a particular
pass, is even harder.  This patch adds debug counters for the individual
back threading passes.  I've left the ethread pass alone, as that one is
usually benign, but we could easily add it if needed.

The fact that we can only pass one boolean argument to the passes
infrastructure has us do all sorts of gymnastics to differentiate
between the various back threading passes.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* dbgcnt.def: Add debug counter for back_thread[12] and
	back_threadfull[12].
	* passes.def: Pass "first" argument to each back threading pass.
	* tree-ssa-threadbackward.c (back_threader::back_threader): Add
	first argument.
	(back_threader::debug_counter): New.
	(back_threader::maybe_register_path): Call debug_counter.

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