Fixed a bug in const folding, where an inst that's not of type

SingleValueInstruction gets added to the worklist, causing cast assert at https://github.com/apple/swift/blame/master/lib/SILOptimizer/Utils/ConstantFolding.cpp#L1585.

One such example inst is the following (in the tensorflow branch), which produces a SILValue of type
MultipleValueInstructionResult, so ValueBase::getDefiningInstruction() still
returns a valid inst for it, even though that graph_op inst is not a SingleValueInstruction.

```
%94 = graph_op "Fill,i,i"(%73 : $TensorHandle<Int32>, %85 : $TensorHandle<Float>) {T: $Float, index_type: $Int32, __device: "/device:CPU:0"} : $TensorHandle<Float>
```

The same fix has been merged into the tensorflow branch: #18272

Author: Mingsheng Hong

lib/SILOptimizer/Utils/ConstantFolding.cpp

@@ -1668,7 +1668,8 @@ ConstantFolder::processWorkList() {
 
       // The new constant could be further folded now, add it to the worklist.
       if (auto *Inst = C->getDefiningInstruction())
-        WorkList.insert(Inst);
+        if (isa<SingleValueInstruction>(Inst))
+          WorkList.insert(Inst);
     }
 
     // Eagerly DCE. We do this after visiting all users to ensure we don't