Moved special casing of 'assert(this, "null this");' generated statements from !ThisExp into !AssertExp.

Fixed filenames for array bounds errors and probably others, fixes #271 .

gen/statements.cpp

@@ -1466,7 +1466,8 @@ void SwitchErrorStatement::toIR(IRState* p)
     std::vector<LLValue*> args;
 
     // file param
-    args.push_back(DtoLoad(gIR->dmodule->ir.irModule->fileName));
+    IrModule* irmod = getIrModule(NULL);
+    args.push_back(DtoLoad(irmod->fileName));
 
     // line param
     LLConstant* c = DtoConstUint(loc.linnum);