path solver: Adjustments for use outside of the backward threader.

path solver: Adjustments for use outside of the backward threader.

Here are some enhancements to make it easier for other clients to use
the path solver.

First, I've made the imports to the solver optional since we can
calculate them ourselves.  However, I've left the ability to set them,
since the backward threader adds a few SSA names in addition to the
default ones.  As a follow-up I may move all the import set up code
from the threader to the solver, as the extra imports tend to improve
the behavior slightly.

Second, Richi suggested an entry point where you just feed the solver
an edge, which will be quite convenient for a subsequent patch adding
a client in the header copying pass.  The required some shuffling,
since we'll be adding the blocks on the fly.  There's now a vector
copy, but the impact will be minimal, since these are just 5-6 entries
at the most.

Tested on ppc64le Linux.

gcc/ChangeLog:

	* gimple-range-path.cc (path_range_query::path_range_query): Do
	not init m_path.
	(path_range_query::dump): Change m_path uses to non-pointer.
	(path_range_query::defined_outside_path):  Same.
	(path_range_query::set_path): Same.
	(path_range_query::add_copies_to_imports): Same.
	(path_range_query::range_of_stmt): Same.
	(path_range_query::compute_outgoing_relations): Same.
	(path_range_query::compute_ranges): Imports are now optional.
	Implement overload that takes an edge.
	* gimple-range-path.h (class path_range_query): Make imports
	optional for compute_ranges.  Add compute_ranges(edge) overload.
	Make m_path an auto_vec instead of a pointer and adjust
	accordingly.