PeepholeOpt: Handle subregister compose when looking through reg_sequence (#127051)

Previously this would give up on folding subregister copies through
a reg_sequence if the input operand already had a subregister index.
d246cc618adc52fdbd69d44a2a375c8af97b6106 stopped introducing these
subregister uses, and this is the first step to lifting that restriction.

I was expecting to be able to implement this only purely with compose /
reverse compose, but I wasn't able to make it work so relies on testing
the lanemasks for whether the copy reads a subset of the input.
4 files changed
tree: 2ffade440ab5e71ddd256e7f6fc05cbd9d40fd17
  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. libunwind/
  16. lld/
  17. lldb/
  18. llvm/
  19. llvm-libgcc/
  20. mlir/
  21. offload/
  22. openmp/
  23. polly/
  24. pstl/
  25. runtimes/
  26. third-party/
  27. utils/
  28. .clang-format
  29. .clang-tidy
  30. .git-blame-ignore-revs
  31. .gitattributes
  32. .gitignore
  33. .mailmap
  34. CODE_OF_CONDUCT.md
  35. CONTRIBUTING.md
  36. LICENSE.TXT
  37. pyproject.toml
  38. README.md
  39. SECURITY.md
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