analyzer: factor out new class feasibility_state

analyzer: factor out new class feasibility_state

As preparatory work for a fix to PR analyzer/96374, this patch
moves the core state-update logic from the loop in
exploded_path::feasible_p into a new class feasibility_state.

No functional change intended.

gcc/analyzer/ChangeLog:
	PR analyzer/96374
	* engine.cc (exploded_path::feasible_p): Move "snodes_visited" and
	"model" locals into a new class feasibility_state.  Move heart
	of per-edge processing into
	feasibility_state::maybe_update_for_edge.
	(feasibility_state::feasibility_state): New.
	(feasibility_state::maybe_update_for_edge): New, based on loop
	body in exploded_path::feasible_p.
	* exploded-graph.h (class feasibility_state): New.