Improve backwards threader debugging dumps.

Improve backwards threader debugging dumps.

This patch adds debugging helpers to the backwards threader.  I have
also noticed that profitable_path_p() can bail early on paths that
crosses loops and leave the dump of blocks incomplete.  Fixed as
well.

Unfortunately the new methods cannot be marked const, because we call
the solver's dump which is not const.  I believe this was because the
ranger dump calls m_cache.block_range().  This could probably use a
cleanup at a later time.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* tree-ssa-threadbackward.c (back_threader::dump): New.
	(back_threader::debug): New.
	(back_threader_profitability::profitable_path_p): Dump blocks
	even if we are bailing early.