Add --param max-unswitch-depth

Add --param max-unswitch-depth

The following adds a --param to limit the depth of unswitched loop
nests.  One can use --param max-unswitch-depth=1 to disable unswitching
of outer loops (the innermost loop will then be unswitched).

	PR tree-optimization/107946
	* params.opt (-param=max-unswitch-depth=): New.
	* doc/invoke.texi (--param=max-unswitch-depth): Document.
	* tree-ssa-loop-unswitch.cc (init_loop_unswitch_info): Honor
	--param=max-unswitch-depth