[llvm] Add comment about the structure of the CodeGen (#6150)
[llvm] Add comment about the structure of the CodeGen (#6150)

Issue: #5511 

### Brief Summary

Co-authored-by: pre-commit-ci[bot] <3ea08453efb076ca910552532a53973a6527df40@users.noreply.github.com>
Co-authored-by: Ailing  <1b6840d5aad3f67904e2d5dd1e336090078ee11e@users.noreply.github.com>
yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi