Update naga benchmarks and add a compaction benchmark

[andyleiserson]

Updates the path to WGSL snapshots in the benchmarks, adds checks that the input set is not empty, and adds a compaction benchmark that I used for the performance data in #7703 (comment).

Testing
Ran the benchmarks.

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.