[XLA:GPU] [NFC] Merge flag parsing logic from hlo_runner_flags into hlo_runner_main
[XLA:GPU] [NFC] Merge flag parsing logic from hlo_runner_flags into hlo_runner_main

Having it in a separate file just makes it non-obvious which flags are used (as
there's only one user), and complicates error handling. Furthermore, it hides
the fact that some flags are duplicated.
