Make Instruction::update_branch_offset usable by both FuncTranlator backends

[Robbepop]

This allows to avoid code duplication and simplifies integration of #1512.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.23%. Comparing base (ae9496b) to head (5c19cff).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/src/engine/translator/comparator.rs	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1560      +/-   ##
==========================================
- Coverage   72.25%   72.23%   -0.03%     
==========================================
  Files         179      179              
  Lines       16333    16331       -2     
==========================================
- Hits        11801    11796       -5     
- Misses       4532     4535       +3     

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