[enhancement] Add `worst_case_throughput` minimization results to benchmark reports

### What's hard to do? (limit 100 words)

[`worst_case_throughput` minimization](https://github.com/google/xls/blob/65aa30d29a36e34040707b7eaf4a8dbb5e06651d/xls/scheduling/run_pipeline_schedule.cc#L551) does not report the actual throughput it achieved in a visible way.
This makes it hard to evaluate performance, especially in `multi_proc` codegen.
Without this information, it is difficult to understand the real performance of the design.

### Current best alternative workaround (limit 100 words)

Generate each proc separately with strict constraints on `worst_case_throughput`.
This ensures the required throughput is met, but adds extra steps and complexity to the workflow.

Additionally, one can increase verbosity of the logs and read it on stdout:
https://github.com/google/xls/blob/65aa30d29a36e34040707b7eaf4a8dbb5e06651d/xls/scheduling/run_pipeline_schedule.cc#L567

### Your view of the "best case XLS enhancement" (limit 100 words)

Extend the `opt_ir_benchmark` report to include the achieved `worst_case_throughput` from the minimization pass.
Showing this value next to the pipeline scheduling data would make it much easier to assess whether performance goals were met.