[SSAUpdater] Avoid un-necessary SmallVector stores (#97820)

The default template for the generic IDF calculator fetching
block-children will, by default:
 * Fetch the children range from the relevant `GraphTraits`,
 * Store all child nodes in a `SmallVector`,
 * Return that `SmallVector` into the IDF calculator.

The only place this `SmallVector` is used is in a for-range loop... thus
there's no reason why we can't just iterate over the child range from
`GraphTraits`, instead of storing all the nodes locally then iterating
over the local copy. (If the children of a node change during IDF
calculation, everything is broken anyway).

This yields a 0.14% debug-info build performance improvement on the
compile time tracker, as InstrRefBasedLDV uses the SSA updater
intensively on all functions.
1 file changed
tree: 6577c4c39ed8e69ab8fc8034f7918f1d69297251
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. libc/
  11. libclc/
  12. libcxx/
  13. libcxxabi/
  14. libunwind/
  15. lld/
  16. lldb/
  17. llvm/
  18. llvm-libgcc/
  19. mlir/
  20. offload/
  21. openmp/
  22. polly/
  23. pstl/
  24. runtimes/
  25. third-party/
  26. utils/
  27. .clang-format
  28. .clang-tidy
  29. .git-blame-ignore-revs
  30. .gitattributes
  31. .gitignore
  32. .mailmap
  33. CODE_OF_CONDUCT.md
  34. CONTRIBUTING.md
  35. LICENSE.TXT
  36. pyproject.toml
  37. README.md
  38. SECURITY.md
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