[SCCP] Only track returns of functions with non-void ret ty (NFC).

There is no need to add functions with void return types to the set of
tracked return values. This does not change functionality, because we
such functions do not have return values and we never update or access
them.

llvm/lib/Transforms/Scalar/SCCP.cpp

@@ -233,7 +233,7 @@ public:
       for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
         TrackedMultipleRetVals.insert(
             std::make_pair(std::make_pair(F, i), ValueLatticeElement()));
-    } else
+    } else if (!F->getReturnType()->isVoidTy())
       TrackedRetVals.insert(std::make_pair(F, ValueLatticeElement()));
   }