[llvm] Add the "initializes" attribute inference (PR #97373)

[Arthur Eubanks via llvm-commits]

https://github.com/aeubanks commented:

even when lgtm, we should land this only after the DSE patch lands as to not unnecessarily increase compile time without any benefit

can you add `[FunctionAttrs]` to the beginning of the commit title?

https://github.com/llvm/llvm-project/pull/97373