[Analyzer] Minor cleanups in StreamChecker

[mantognini]

Summary

Remove unnecessary conversion to Optional<> and incorrect assumption
that BindExpr can return a null state.

[mantognini]

For reference, the modified code was introduced with https://reviews.llvm.org/D80015.

Any feedback is appreciated.

[steakhal]

Looks great, thanks. Land it.

clang/lib/StaticAnalyzer/Checkers/StreamChecker.cpp

if (State->isNull(*SizeVal).isConstrainedTrue() ||
     return;
   }
 
   // Generate a transition for the success state.
   // If we know the state to be FEOF at fread, do not add a success state.
   if (!IsFread || (OldSS->ErrorState != ErrorFEof)) {
     ProgramStateRef StateNotFailed =
         State->BindExpr(CE, C.getLocationContext(), *NMembVal);
-    if (StateNotFailed) {
-      StateNotFailed = StateNotFailed->set<StreamMap>(
-          StreamSym, StreamState::getOpened(Desc));
-      C.addTransition(StateNotFailed);
-    }
-  }
+    StateNotFailed =
+        StateNotFailed->set<StreamMap>(StreamSym, StreamState::getOpened(Desc));
 
   // Add transition for the failed state.
-  Optional<NonLoc> RetVal = makeRetVal(C, CE).castAs<NonLoc>();
-  assert(RetVal && "Value should be NonLoc.");
+  NonLoc RetVal = makeRetVal(C, CE).castAs<NonLoc>();
   ProgramStateRef StateFailed =
-      State->BindExpr(CE, C.getLocationContext(), *RetVal);
-  if (!StateFailed)
-    return;
-  auto Cond = C.getSValBuilder()
-                  .evalBinOpNN(State, BO_LT, *RetVal, *NMembVal,
-                               C.getASTContext().IntTy)
-                  .getAs<DefinedOrUnknownSVal>();
+      State->BindExpr(CE, C.getLocationContext(), RetVal);
+  auto Cond =
+      C.getSValBuilder()
+          .evalBinOpNN(State, BO_LT, RetVal, *NMembVal, C.getASTContext().IntTy)
   if (!Cond)
     return;
   StateFailed = StateFailed->assume(*Cond, true);
   if (!StateFailed)
     return;
 
   StreamErrorState NewES;
   if (IsFread)