Improve V3Premit performance etc. (#4736)

- Enable creating constant pool entries for RHS of simple
  var = const assignments
- Never extract ArraySel (it's just pointer arithmetic)
- Remove unnecessary AstTraceInc precond child tree
- Always fully recurse expressions (fix transplanted from #4617)
- General cleanup

Overall the patch is performance neutral to slightly positive, but saves
~10% peak Verialtor memory usage due to not creating temporaries (which
are later expanded) for any ArraySels.

src/V3EmitCImp.cpp

@@ -740,7 +740,6 @@ class EmitCTrace final : EmitCFunc {
     }
 
     void emitTraceChangeOne(AstTraceInc* nodep, int arrayindex) {
-        iterateAndNextConstNull(nodep->precondsp());
         // Note: Both VTraceType::CHANGE and VTraceType::FULL use the 'full' methods
         const std::string func = nodep->traceType() == VTraceType::CHANGE ? "chg" : "full";
         bool emitWidth = true;