Strengthen assertions further

Author: DougGregor

lib/Sema/CSGen.cpp

@@ -3947,9 +3947,10 @@ namespace {
 
     Type visitTapExpr(TapExpr *expr) {
       DeclContext *varDC = expr->getVar()->getDeclContext();
+      ASSERT(varDecl != nullptr);
       ASSERT((varDC == CS.DC ||
-              (varDC && isa<AbstractClosureExpr>(varDC)) ||
-              (varDC && varDC->isChildContextOf(CS.DC))) &&
+              isa<AbstractClosureExpr>(varDC) ||
+              varDC->isChildContextOf(CS.DC)) &&
              "TapExpr var should be in the same DeclContext we're checking it in!");
 
       auto locator = CS.getConstraintLocator(expr);