SwitchComposite GC.Alloc every OnStart

Steps to reproduce:

1. Create a new graph, a new enum and a Switch node
2. Create a BBV for the enum and assign it to the Switch node
3. Connect the Switch node to the Start node, and create and connect a LogMessage to the first Switch port.
4. Attach the graph to a GameObject
5. Enter playmode and profile the code (Profiler, Deep Profile mode, GC and Timeline markers)

Actual results: 

• Observe that SwitchComposite is creating GC.Alloc every frame

Expected results: 

• SwitchComposite doesn't GC.Alloc every frame.

Reproducible with versions: 
1.0.0 - 1.0.11

Notes:

•  SwitchComposite should cache enum value the first time.