The compare_perf_reports script should support an optional --use_trace_diff flag.

• Inputs: Accept either traces or perf reports.
• Trace mode: When traces are provided, generate perf reports and optionally run TraceDiff to add extra comparison tables.
• Report mode: When perf reports are provided directly, run the standard comparison as today.
• Number of inputs: Restrict to two (rarely more than two are needed, and this fits better with TraceDiff).
• Other args: Keep the existing arguments unchanged.