Enable ExistentialSpecializer by default

Author: Raj Barik

include/swift/AST/SILOptions.h

@@ -51,7 +51,7 @@ class SILOptions {
   bool RemoveRuntimeAsserts = false;
 
   /// Enable existential specializer optimization.
-  bool ExistentialSpecializer = false;
+  bool ExistentialSpecializer = true;
 
   /// Controls whether the SIL ARC optimizations are run.
   bool EnableARCOptimizations = true;