Make cut_loop work on loops with a variable upper bound

[yamaguchi1024]

Closes #235

[alexreinking]

This will conflict with the changes I'm making to convert the primitive directives to use cursors.

[codecov-commenter]

Codecov Report

Merging #315 (f98adf8) into master (b01b3be) will increase coverage by 0.11%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master     #315      +/-   ##
==========================================
+ Coverage   86.26%   86.38%   +0.11%     
==========================================
  Files          73       73              
  Lines       16611    16625      +14     
==========================================
+ Hits        14329    14361      +32     
+ Misses       2282     2264      -18     

Impacted Files	Coverage Δ
src/exo/API_scheduling.py	87.07% <100.00%> (+0.68%)	⬆️
src/exo/LoopIR_scheduling.py	90.68% <100.00%> (+0.87%)	⬆️
tests/test_schedules.py	99.02% <100.00%> (+0.02%)	⬆️
tests/conftest.py	82.27% <0.00%> (-3.80%)	⬇️

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