Migrate benchmarks to MergeQueue

[sarahchen6]

What Does This Do

Run startup, load, and dacapo benchmarks on the merge queue instead of for every PR push. Allow these benchmarks to be run with manual triggers or by setting the CI variable BENCHMARKS=true.

Motivation

By migrating these benchmarks to the merge queue, we can greatly reduce CI runtime (from ~60 min to ~35 min) and support faster development experiences. At the same time, however, we don't want to lose benchmarking ability nor merge in unknown regressions.

Additional Notes

We should ensure existing SLOs to block PRs that introduce major regressions are updated. We can also consider adding a line in the default PR description to suggest manually running benchmarks before adding to the merge queue.

Contributor Checklist

• Format the title according the contribution guidelines
• Assign the type: and (comp: or inst:) labels in addition to any useful labels
• Don't use close, fix or any linking keywords when referencing an issue.
  Use solves instead, and assign the PR milestone to the issue
• Update the CODEOWNERS file on source file addition, move, or deletion
• Update the public documentation in case of new configuration flag or behavior

Jira ticket: [PROJ-IDENT]