[SimpleLoopUnswitch][NFC] move quadratic asserts under EXPENSIVE_CHECKS (#144887)

Three asserts/checks in SimpleLoopUnswitchPass are quite expensive on IR
containing many deeply nested loop nests.
Their cost is not linear with the number of loop nests and these asserts
quickly become the most significant cost of the whole compilation.

This patch move some of the asserts under `EXPENSIVE_CHECKS` and leaves the one
line 1080 unguarded. 

This problem was exposed with flang because of Fortran
multidimensional arrays and array expressions. In some programs, half of the end to
end compilation time was spent in those asserts.
1 file changed
tree: 3d5a0939cb53c78caf4362516215b8ff31ad1725
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libsycl/
  16. libunwind/
  17. lld/
  18. lldb/
  19. llvm/
  20. llvm-libgcc/
  21. mlir/
  22. offload/
  23. openmp/
  24. orc-rt/
  25. polly/
  26. runtimes/
  27. third-party/
  28. utils/
  29. .clang-format
  30. .clang-format-ignore
  31. .clang-tidy
  32. .git-blame-ignore-revs
  33. .gitattributes
  34. .gitignore
  35. .mailmap
  36. CODE_OF_CONDUCT.md
  37. CONTRIBUTING.md
  38. LICENSE.TXT
  39. pyproject.toml
  40. README.md
  41. SECURITY.md
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