Reapply [Metadata] Preserve MD_prof when merging instructions when one is missing. (#135418)

Preserve branch weight metadata when merging instructions if one of the
instructions is missing metadata. This is similar in behaviour to what
we do today for other types of metadata such as mmra, memprof and
callsite metadata.

Also add a legality check when merging prof metadata based on
instruction type. Without this check GVN PRE optimizations result in
prof metadata on phi nodes which break the module verifier.

Build failure caught by
https://lab.llvm.org/buildbot/#/builders/113/builds/6621
```
!9185 = !{!"branch_weights", i32 3912, i32 802}
Wrong number of operands
!9185 = !{!"branch_weights", i32 3912, i32 802}
fatal error: error in backend: Broken module found, compilation aborted!
```

Reverts #134200 with additional changes.
5 files changed
tree: 8fc9c8ff2fc7a1f81ba3252340506a1ba7ecbf7e
  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-format-ignore
  30. .clang-tidy
  31. .git-blame-ignore-revs
  32. .gitattributes
  33. .gitignore
  34. .mailmap
  35. CODE_OF_CONDUCT.md
  36. CONTRIBUTING.md
  37. LICENSE.TXT
  38. pyproject.toml
  39. README.md
  40. SECURITY.md
README.md

The LLVM Compiler Infrastructure

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Welcome to the LLVM project!

This repository contains the source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and run-time environments.

The LLVM project has multiple components. The core of the project is itself called “LLVM”. This contains all of the tools, libraries, and header files needed to process intermediate representations and convert them into object files. Tools include an assembler, disassembler, bitcode analyzer, and bitcode optimizer.

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