path solver: Only compute relations for imports.

path solver: Only compute relations for imports.

We are currently calculating implicit PHI relations for all PHI
arguments.  This creates unecessary work, as we only care about SSA
names in the import bitmap.  Similarly for inter-path relationals.  We
can avoid things not in the bitmap.

Tested on x86-64 and ppc64le Linux with the usual regstrap.  I also
verified that the before and after number of threads was the same
in a suite of .ii files from a bootstrap.

gcc/ChangeLog:

	PR tree-optimization/102943
	* gimple-range-path.cc (path_range_query::compute_phi_relations):
	Only compute relations for SSA names in the import list.
	(path_range_query::compute_outgoing_relations): Same.
	* gimple-range-path.h (path_range_query::import_p): New.