[naga] Allow unreachable statements after return/break/continue/discard

[andyleiserson]

Fixes #7536

This is an alternative to #7554, which generates a warning on unreachable statements. Reasons to take this simpler change instead are that the diagnostic infrastructure is not mature yet, and it is likely that eventual fixes to uniformity analysis (#4369) will also need to consider unreachable statements, so it may make more sense to emit an unreachable statements diagnostic from within the uniformity analysis implementation.

Testing
Adds a test that an unreachable statement after return is permitted and still contributes to global use. Also enables some relevant CTS tests.

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.