Remove TDF_THREADING flag in favor of param.

Remove TDF_THREADING flag in favor of param.

I am returning a TDF_* flag to the queue of available entries as I am
unconvinced that we need to burn an entire flag for internal debugging
constructs, especially since we seem to be running out of them.

I've added a --param=threader-debug entry similar to the one we use for
ranger debugging.  Currently this only affects the backward threader,
but since the DOM threader is an outlier and on the chopping block, I
avoided using the "backward" name.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* dumpfile.c (dump_options): Remove TDF_THREADING entry.
	* dumpfile.h (enum dump_flag): Remove TDF_THREADING and adjust
	remaining entries.
	* flag-types.h (enum threader_debug): New.
	* gimple-range-path.cc (DEBUG_SOLVER): Use param_threader_debug.
	* params.opt: Add entry for --param=threader-debug=.