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/V3EmitCFunc.cpp

@@ -32,7 +32,7 @@ constexpr int VL_VALUE_STRING_MAX_WIDTH = 8192;
 //######################################################################
 // EmitCFunc
 
-bool EmitCFunc::emitSimpleOk(AstNodeMath* nodep) {
+bool EmitCFunc::emitSimpleOk(AstNodeExpr* nodep) {
     // Can we put out a simple (A + B) instead of VL_ADD_III(A,B)?
     if (nodep->emitSimpleOperator() == "") return false;
     if (nodep->isWide()) return false;
@@ -433,12 +433,7 @@ void EmitCFunc::emitCCallArgs(const AstNodeCCall* nodep, const string& selfPoint
         iterate(subnodep);
         comma = true;
     }
-    if (VN_IS(nodep->backp(), NodeMath) || VN_IS(nodep->backp(), CReturn)) {
-        // We should have a separate CCall for math and statement usage, but...
-        puts(")");
-    } else {
-        puts(");\n");
-    }
+    puts(")");
 }
 
 void EmitCFunc::emitDereference(const string& pointer) {