[ScopInfo] Do not create entries in map if non exists

Suggested-by:  Michael Kruse <llvm@meinersbur.de>
llvm-svn: 308491
diff --git a/polly/lib/Analysis/ScopInfo.cpp b/polly/lib/Analysis/ScopInfo.cpp
index d0247e8..a782930 100644
--- a/polly/lib/Analysis/ScopInfo.cpp
+++ b/polly/lib/Analysis/ScopInfo.cpp
@@ -3746,7 +3746,7 @@
 
 void Scop::removeStmtNotInDomainMap() {
   auto ShouldDelete = [this](ScopStmt &Stmt) -> bool {
-    return !this->DomainMap[Stmt.getEntryBlock()];
+    return !this->DomainMap.lookup(Stmt.getEntryBlock());
   };
   removeStmts(ShouldDelete);
 }