Use SPIR_TWORK_AUTO for automatic precision selection in benchmark

[shinaoka]

Summary

This PR updates to use instead of hardcoded for automatic precision selection.

Changes

• Changed parameter from to
• Automatically selects appropriate precision based on epsilon:
  • (f64) for epsilon >= 1e-8
  • (Df64) for epsilon < 1e-8

Benefits

1. Performance improvement: Avoids unnecessary extended precision overhead when f64 is sufficient
2. Better accuracy: Automatically uses extended precision when needed (epsilon < 1e-8)
3. Matches actual usage: Benchmark now reflects real-world usage patterns where precision is selected based on epsilon

Testing

• Benchmark runs successfully with automatic precision selection
• Performance improved for epsilon >= 1e-8 cases (no xprec overhead)
• Extended precision still used when needed (epsilon < 1e-8)