Make all expressions derive from AstNodeExpr (#3721).

Apart from the representational changes below, this patch renames
AstNodeMath to AstNodeExpr, and AstCMath to AstCExpr.

Now every expression (i.e.: those AstNodes that represent a [possibly
void] value, with value being interpreted in a very general sense) has
AstNodeExpr as a super class. This necessitates the introduction of an
AstStmtExpr, which represents an expression in statement position, e.g :
'foo();' would be represented as AstStmtExpr(AstCCall(foo)). In exchange
we can get rid of isStatement() in AstNodeStmt, which now really always
represent a statement

Peak memory consumption and verilation speed are not measurably changed.

Partial step towards #3420

src/V3DfgAstToDfg.cpp

@@ -39,7 +39,7 @@ VL_DEFINE_DEBUG_FUNCTIONS;
 
 namespace {
 
-// Create a DfgVertex out of a AstNodeMath. For most AstNodeMath subtypes, this can be done
+// Create a DfgVertex out of a AstNodeExpr. For most AstNodeExpr subtypes, this can be done
 // automatically. For the few special cases, we provide specializations below
 template <typename Vertex, typename Node>
 Vertex* makeVertex(const Node* nodep, DfgGraph& dfg) {
@@ -132,7 +132,7 @@ class AstToDfgVisitor final : public VNVisitor {
     }
 
     // Returns true if the expression cannot (or should not) be represented by DFG
-    bool unhandled(AstNodeMath* nodep) {
+    bool unhandled(AstNodeExpr* nodep) {
         // Short-circuiting if something was already unhandled
         if (!m_foundUnhandled) {
             // Impure nodes cannot be represented