Simplify impl_shift_amount macro

[Robbepop]

Pulled out from #1512.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.28%. Comparing base (2510004) to head (d3ac2b2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1550      +/-   ##
==========================================
- Coverage   72.30%   72.28%   -0.03%     
==========================================
  Files         179      179              
  Lines       16401    16401              
==========================================
- Hits        11859    11855       -4     
- Misses       4542     4546       +4     

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