Skip rename_symbolt::rename when maps are empty

[tautschnig]

rename_symbolt::rename(exprt) creates a depth_iterator and walks the entire expression tree even when both expr_map and type_map are empty. This happens during library linking when no symbols need renaming. Add an early return to avoid the wasted traversal.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Avoids an unnecessary full expression-tree traversal in rename_symbolt::rename(exprt&) when there are no renaming rules, improving library-link-time performance in no-op cases.

Changes:

• Add an early return in rename_symbolt::rename(exprt&) when both expr_map and type_map are empty.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.51%. Comparing base (b32074b) to head (43da1bc).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8974      +/-   ##
===========================================
+ Coverage    80.50%   80.51%   +0.01%     
===========================================
  Files         1704     1704              
  Lines       188778   188779       +1     
  Branches        73       73              
===========================================
+ Hits        151975   151995      +20     
+ Misses       36803    36784      -19     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.