Make ranger optional in path_range_query.

Make ranger optional in path_range_query.

All users of path_range_query are currently allocating a gimple_ranger
only to pass it to the query object.  It's tidier to just do it from
path_range_query if no ranger was passed.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* gimple-range-path.cc (path_range_query::path_range_query): New
	ctor without a ranger.
	(path_range_query::~path_range_query): Free ranger if necessary.
	(path_range_query::range_on_path_entry): Adjust m_ranger for pointer.
	(path_range_query::ssa_range_in_phi): Same.
	(path_range_query::compute_ranges_in_block): Same.
	(path_range_query::compute_imports): Same.
	(path_range_query::compute_ranges): Same.
	(path_range_query::range_of_stmt): Same.
	(path_range_query::compute_outgoing_relations): Same.
	* gimple-range-path.h (class path_range_query): New ctor.
	* tree-ssa-loop-ch.c (ch_base::copy_headers): Remove gimple_ranger
	as path_range_query allocates one.
	* tree-ssa-threadbackward.c (class back_threader): Remove m_ranger.
	(back_threader::~back_threader): Same.