Internals: Use and enforce class final for ~5% performance boost.

src/V3GraphPathChecker.h

@@ -29,7 +29,7 @@
 ///
 /// The graph (or at least, the subset the algorithm sees through
 /// edgeFuncp) must not change during the lifetime of the checker.
-class GraphPathChecker : GraphAlg<const V3Graph> {
+class GraphPathChecker final : GraphAlg<const V3Graph> {
     // Count "generations" which increases on operations that scan through
     // the graph. Each node is marked with the last generation that scanned
     // it, to enable asserting there are no cycles, and to avoid recursing