Refactor cmp+select fusion code

[Robbepop]

This is required to simplify integration of #1512.

[codecov]

Codecov Report

Attention: Patch coverage is 80.76923% with 5 lines in your changes missing coverage. Please review.

Project coverage is 72.28%. Comparing base (341847d) to head (966a93f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../wasmi/src/engine/translator/func/instr_encoder.rs	75.00%	3 Missing ⚠️
crates/wasmi/src/engine/translator/comparator.rs	83.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1558   +/-   ##
=======================================
  Coverage   72.28%   72.28%           
=======================================
  Files         179      179           
  Lines       16321    16333   +12     
=======================================
+ Hits        11798    11807    +9     
- Misses       4523     4526    +3     

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