Reset relations when crossing backedges.

As discussed in PR103721, the problem here is that we are crossing a
backedge and causing us to use relations from a previous iteration of a
loop.

This handles the testcases in both PR103721 and PR104067 which are variants
of the same thing.

Tested on x86-64 Linux with the usual regstrap as well as verifying the
thread count before and after the patch.  The number of threads is
reduced by a miniscule amount.

gcc/ChangeLog:

	PR tree-optimization/103721
	* gimple-range-path.cc
	(path_range_query::relations_may_be_invalidated): New.
	(path_range_query::compute_ranges_in_block): Reset relations if
	they may be invalidated.
	(path_range_query::maybe_register_phi_relation): Exit if relations
	may be invalidated on incoming edge.
	(path_range_query::compute_phi_relations): Pass incoming PHI edge
	to maybe_register_phi_relation.
	* gimple-range-path.h (relations_may_be_invalidated): New.
	(maybe_register_phi_relation): Pass edge instead of tree.
	* tree-ssa-threadbackward.cc (back_threader::back_threader):
	Mark DFS edges.
	* value-relation.cc (path_oracle::path_oracle): Call
	mark_dfs_back_edges.
	(path_oracle::register_relation): Add SSA names to m_registered
	bitmap.
	(path_oracle::reset_path): Clear m_registered bitmap.
	* value-relation.h (path_oracle::set_root_oracle): New.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr103721-2.c: New test.
	* gcc.dg/pr103721.c: New test.

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