[llvm] Port Swift's merge function pass to llvm: merging functions that differ in constants (PR #68235)

[Manman Ren via llvm-commits]

manman-ren wrote:

> Please revert this PR, it has not been adequately reviewed.

Feel free to revert it. I am not sure how to yet.
There is also https://github.com/llvm/llvm-project/pull/71219 to fix build issues on top of this PR.
Do I need to create a different PR after the reverting? Or we can continue using this PR for further reviewing?


https://github.com/llvm/llvm-project/pull/68235