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

src/V3AssertPre.cpp

@@ -26,7 +26,7 @@
 //######################################################################
 // Assert class functions
 
-class AssertPreVisitor : public AstNVisitor {
+class AssertPreVisitor final : public AstNVisitor {
     // Removes clocks and other pre-optimizations
     // Eventually inlines calls to sequences, properties, etc.
     // We're not parsing the tree, or anything more complicated.