[Synth] Replace terminology `fan_in/fan_out` with `start_point/end_point` in LongestPathAnalysis

[uenoku]

fan-in/out refer to incoming/outcoming edge for each node and start_point/end_point would be more standard way to represent first/last element of a path.

[markram1729]

Hi @uenoku , Can you please check my pr if this resolves the issue