[RISCV] Refactor RVV builtin code generation for reduce compilation time [NFC] (#154906)

Extract ManualCodegen blocks from riscv_vector.td to dedicated helper
functions in RISCV.cpp to improve compilation times and code
organization.

This refactoring:
- Reduces riscv_vector_builtin_cg.inc from ~70,000 lines to ~30,000
lines
- Extracts lots of ManualCodegen blocks into helper functions in
RISCV.cpp
- Moves complex code generation logic from TableGen to C++
- Marks extracted functions with LLVM_ATTRIBUTE_NOINLINE to prevent
excessive inlining in EmitRISCVBuiltinExpr's large switch statement,
which would cause compilation time to increase significantly

Performance Impact on AMD Ryzen 9 3950X 16-Core with SSD (Release build)
with GCC 11:

Before: real 1m4.560s, user 0m0.529s, sys 0m0.175s
After:  real 0m22.577s, user 0m0.498s, sys 0m0.152s

Which reduced around 65% of compilation time.

During this refactoring, I also found few more opportunities to optimize
and simplify the code generation logic, but I think leave to next PR
since it already change a lot of code.

Fix #88368
2 files changed
tree: 2ff483601875b77bdd90e28c735747d760aa9a68
  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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