[semantic-arc-opts] Fix a thinko.

This can only affect us in code where we use address phi arguments (which can
never happen in raw sil where this runs today).

I am going to be moving it later once I finish bringing up lowering ownership
after diagnostics.

Author: gottesmm

lib/SILOptimizer/Mandatory/SemanticARCOpts.cpp

@@ -369,7 +369,7 @@ static bool isWrittenTo(SILFunction &f, SILValue value) {
   // conservative and assume that the value is written to.
   const auto &storage = findAccessedStorageNonNested(value);
   if (!storage)
-    return false;
+    return true;
 
   // Then see if we ever write to this address in a flow insensitive
   // way (ignoring stores that are obviously the only initializer to