Add --param=dom-jump-threading to disable DOM jump threading [PR126103]

Add --param=dom-jump-threading to disable DOM jump threading [PR126103]

As discussed in PR126103, the first step to removing DOM is untangling
its threader from the rest of DOM.  I've started auditing what we're
missing in PRE + backwards threader, and need a way to disable DOM's
threading for the barrage of PRs I'm about to file.

We already have a way to disable all jump threads, but no way to
disable just DOM's.  This patch does jhust this with
--param=dom-jump-threading=[01].  It's in line with what we
temporarily did for ranger-threading and ranger VRP.

The --param disables jump threading in DOM, but everything else DOM
does is unaffected.

Tested on ppc64le Linux.

	PR tree-optimization/126103

gcc/ChangeLog:

	* doc/params.texi (dom-jump-threading): Document.
	* params.opt (-param=dom-jump-threading=): New.
	* tree-ssa-dom.cc (dom_opt_dom_walker::after_dom_children): Honor
	param_dom_jump_threading.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/dom-jump-threading-1.c: New test.
	* gcc.dg/tree-ssa/dom-jump-threading-2.c: New test.