Fix PHI ordering problems in the path solver.

After auditing the PHI range calculations, I'm not convinced we've
caught all the corner cases.  They haven't shown up in the wild (yet),
but better safe than sorry.

We shouldn't write anything to the cache or trigger additional
lookups while calculating a PHI, as this may cause ordering problems.
We should resolve the PHI with either the cache as it stands, or by
asking for ranges on entry to the path.  I've documented this.

There was one dubious case where we called fold_range in
ssa_range_in_phi, which mostly by luck wasn't triggering lookups,
because fold_range solves a PHI by calling range_on_edge, which is set
to pick up global ranges by default in path_range_query.  This is
fragile, so I've rewritten the call to explicitly use cached or global
ranges.

Also, the cache should be avoided in ssa_range_in_phi when the arg is
defined in the PHI's block, as not doing so could create an ordering
problem.  We have a similar check when calculating relations in PHIs.

Tested on x86-64 & ppc64le Linux.

gcc/ChangeLog:

	* gimple-range-path.cc (path_range_query::internal_range_of_expr):
	Remove useless code.
	(path_range_query::ssa_defined_in_bb): New.
	(path_range_query::ssa_range_in_phi): Avoid fold_range call that
	could trigger additional lookups.
	Do not use the cache for ARGs defined in this block.
	(path_range_query::compute_ranges_in_block): Use ssa_defined_in_bb.
	(path_range_query::maybe_register_phi_relation): Same.
	(path_range_query::range_of_stmt): Adjust comment.
	* gimple-range-path.h (ssa_defined_in_bb): New.

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