Readability improvement on graph_properties:

- Use 'ic' for InferenceContext*, and 'ctx' for NodeContext* to be consistent within graph_properties (there are places where 'c' is used for either one in short functions).
- The inner loop for iterating over input nodes in UpdateNode is redundant, and thus removed.
- UpdateMergeNode is renamed to UpdateMerge in consistence with other Update functions;
- Some other minor changes.

PiperOrigin-RevId: 228972916