Make constant_propagation_08 test platform agnostic

[tautschnig]

We had a varying number of simplifications as platforms with sizeof(int) != sizeof(long) would yield 2 simplifications, and those with matching bit widths only a single one. The extra simplification referred to the cast in the index operand (on platforms with sizeof(int) != sizeof(long)), which we can avoid by making the index a long-type constant in the first place.

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (e02fd74) 78.94% compared to head (7b3e39e) 79.01%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7912      +/-   ##
===========================================
+ Coverage    78.94%   79.01%   +0.07%     
===========================================
  Files         1697     1697              
  Lines       195379   195379              
===========================================
+ Hits        154242   154386     +144     
+ Misses       41137    40993     -144     

see 8 files with indirect coverage changes

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[martin-cs]

Thanks!