path solver: Add related SSAs to solvable set.

path solver: Add related SSAs to solvable set.

The path solver takes an initial set of SSA names which are deemed
interesting.  These are then solved along the path.  Adding any copies
of said SSA names to the list of interesting names yields significantly
better results.  This patch adds said copies to the already provided
list.

Currently this code is guarded by "m_resolve", which is the more
expensive mode, but it would be reasonable to make it available always,
especially since adding more imports usually has minimal impact on the
processing time.  I will investigate and make it universally available
if this is indeed the case.

gcc/ChangeLog:

	* gimple-range-path.cc (path_range_query::add_to_imports): New.
	(path_range_query::add_copies_to_imports): New.
	(path_range_query::precompute_ranges): Call
	add_copies_to_imports.
	* gimple-range-path.h (class path_range_query): Add prototypes
	for add_copies_to_imports and add_to_imports.