Reorder relation calculating code in the path solver.

Enabling the fully resolving threader triggers various relation
ordering issues that have previously been dormant because the VRP
hybrid threader (forward threader based) never gives us long enough
paths for this to matter.  The new threader spares no punches in
finding non-obvious paths, so getting the relations right is
paramount.

This patch fixes a couple oversights that have gone undetected.

First, some background.  There are 3 types of relations along a path:

a) Relations inherent in a PHI.
b) Relations as a side-effect of evaluating a statement.
c) Outgoing relations between blocks in a path.

We must calculate these in their proper order, otherwise we can run
into ordering issues.  The current ordering is wrong, as we
precalculate PHIs for _all_ blocks before anything else, and then
proceed to register the relations throughout the path.  Also, we fail
to realize that a PHI whose argument is also defined in the PHIs block
cannot be registered as an equivalence without causing more ordering
issues.

This patch fixes all the problems described above.  With it we get a
handful more net threads, but most importantly, we disallow some
threads that were wrong.

Tested on x86-64 and ppc64le Linux on the usual regstrap, plus by
comparing the different thread counts before and after this patch.

gcc/ChangeLog:

	* gimple-range-fold.cc (fold_using_range::range_of_range_op): Dump
	operands as well as relation.
	* gimple-range-path.cc
	(path_range_query::compute_ranges_in_block): Compute PHI relations
	first.  Compute outgoing relations at the end.
	(path_range_query::compute_ranges): Remove call to compute_relations.
	(path_range_query::compute_relations): Remove.
	(path_range_query::maybe_register_phi_relation): New.
	(path_range_query::compute_phi_relations): Abstract out
	registering one PHI relation to...
	(path_range_query::compute_outgoing_relations): ...here.
	* gimple-range-path.h (class path_range_query): Remove
	compute_relations.
	Add maybe_register_phi_relation.

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