Revert the avoid threading circular paths commit.

Revert the avoid threading circular paths commit.

I've tested this patch on the wrong tree, and picked up the test changes
in a pending patch, without which this patch is no longer obvious.
Plus, it causes a regression in an invalid test I've recommended we remove.

I'm reverting this patch until the dependencies are reviewed.

Sorry for the noise.

gcc/ChangeLog:

	* tree-ssa-threadbackward.c
	(back_threader::maybe_register_path): Remove circular paths check.