remove_returns: do not lose location numbers

[tautschnig]

Fully replacing an instruction would also invalidate its location number. This isn't necessary here, and we also didn't run compute_location_numbers after this change.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f034dbb) 79.69% compared to head (a235b44) 79.69%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8178   +/-   ##
========================================
  Coverage    79.69%   79.69%           
========================================
  Files         1680     1680           
  Lines       195242   195242           
========================================
  Hits        155604   155604           
  Misses       39638    39638           

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