Fix issue #2176

[XiaowenHu96]

This PR fixes #2176

The issue here is that equivalence relations in the interpreter will still use as many indices as index-analysis suggests.
However, extend in the interpreter was performed only on the main indices of the two relations.
When an equivalence relation has multiple indices, extend operation needs to be performed from source to every single index in target in order to make sure all indices agree.

[codecov]

Codecov Report

Merging #2178 (9bc5da0) into master (ab8929f) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2178      +/-   ##
==========================================
- Coverage   75.55%   75.53%   -0.02%     
==========================================
  Files         452      452              
  Lines       27249    27250       +1     
==========================================
- Hits        20587    20584       -3     
- Misses       6662     6666       +4     

Impacted Files	Coverage Δ
src/interpreter/Relation.h	90.56% <100.00%> (+0.08%)	⬆️
...ouffle/datastructure/ConcurrentInsertOnlyHashMap.h	88.18% <0.00%> (-1.82%)	⬇️
src/include/souffle/utility/ParallelUtil.h	83.72% <0.00%> (-1.56%)	⬇️