[XLA] Ergonomic improvements to --xla_hlo_profile.

- Don't display ops with 0 optimal seconds and 0 actual cycles.  These
  are ops that were expected to be free and were actually free.

- Fix HloCostAnalysis to mark parameters, constants, and
  get-tuple-element as expected-to-be-free per the definition above.

- Allow optimal-seconds < 0 to indicate "I don't know".  Use this for
  custom calls, and then hide such ops from the "seconds above the
  optimum" table.

- Don't display "<none>" and "<unknown>" -- instead, just display the
  empty string.  Less visual noise.

- Instead of showing ~5 ops per category in the categories tables, show
  everything.  This isn't so noisy now that we're hiding "free" ops, and
  it makes finding optimization opportunities much easier.

PiperOrigin-RevId: 196564177