Add .lower() methods to reduction and replication expressions

[kroening]

Add lower() methods to reduction_and_exprt, reduction_nand_exprt, reduction_or_exprt, reduction_nor_exprt, reduction_xor_exprt, reduction_xnor_exprt, and replication_exprt in bitvector_expr.h/bitvector_expr.cpp.

These methods lower high-level bitvector reduction and replication operators to simpler expression forms:

• reduction_and(a) → (a = all_ones)
• reduction_nand(a) → (a != all_ones)
• reduction_or(a) → (a != 0)
• reduction_nor(a) → (a = 0)
• reduction_xor(a) → a[0] ^ a[1] ^ ... ^ a[n-1]
• reduction_xnor(a) → !(a[0] ^ a[1] ^ ... ^ a[n-1])
• replication(n, x) → x :: x :: ... :: x (n times)

Use the new .lower() methods in smt2_conv.cpp to replace inline lowering code for reduction_and, reduction_nand, reduction_or, reduction_nor, and reduction_xnor. The reduction_xor case retains its native SMT-LIB encoding using extract + bvxor for efficiency.

The boolbv layer is not changed since it operates at the propositional literal level, and the SMT2 replication keeps its native repeat operator.

Companion to diffblue/hw-cbmc#2037 which will use these methods via .lower() instead of free functions.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.82%. Comparing base (f71fdad) to head (67842c6).

Files with missing lines	Patch %	Lines
src/util/bitvector_expr.cpp	43.24%	21 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9123      +/-   ##
===========================================
- Coverage    80.83%   80.82%   -0.01%     
===========================================
  Files         1715     1715              
  Lines       189948   189979      +31     
  Branches        73       73              
===========================================
+ Hits        153540   153547       +7     
- Misses       36408    36432      +24     

☔ View full report in Codecov by Harness.
📢 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.