BandIndex and inbounds-propagation in diagzero

[jishnub]

Using BandIndex helps remove the branches in the indexing operation, and directly index into the diagonal band.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.75%. Comparing base (ea8e858) to head (4763c49).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1347      +/-   ##
==========================================
- Coverage   93.75%   93.75%   -0.01%     
==========================================
  Files          34       34              
  Lines       15761    15761              
==========================================
- Hits        14777    14776       -1     
- Misses        984      985       +1     

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