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

[Haopeng Liu via llvm-commits]

haopliu wrote:

Updated this PR according to the latest comments. Here is the final compile time result (similar as our previous experiments):
- This PR VS. the base: +0.1%, [link](https://llvm-compile-time-tracker.com/compare.php?from=7b88e7530d4329ff0c7c8638f69b39fa1e540218&to=896c638f874b0523f8a4b30e18e6805f3f10829e&stat=instructions%3Au)
- Enable DSE VS. this PR: ~0, [link](https://llvm-compile-time-tracker.com/compare.php?from=896c638f874b0523f8a4b30e18e6805f3f10829e&to=46d896bc578468f608cbe318b1528d8f740e1bc1&stat=instructions%3Au) 

I initially plan to merge this PR one week later if no other comments. Please take a look and feel free to let me know. Thank you all! 

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