Fix nemo warnings about undefined $this

The fix for the crash caused us to take a different path
when checking locals.

hphp/compiler/expression/simple_variable.cpp

@@ -172,7 +172,7 @@ void SimpleVariable::analyzeProgram(AnalysisResultPtr ar) {
       }
     }
   } else if (ar->getPhase() == AnalysisResult::AnalyzeFinal) {
-    if (m_sym) {
+    if (m_sym && !m_this) {
       if (!m_sym->isSystem() &&
           !(getContext() &
             (LValue|RefValue|RefParameter|UnsetContext|ExistContext)) &&