[llvm] Add the 'initializes' attribute langref and support (PR #84803)

[Haopeng Liu via llvm-commits]

haopliu wrote:

Here is an updated compile time regression of the entire stack of patches: 
   this current RP + attribute inference with a dataflow approach ([53bd30](https://github.com/llvm/llvm-project/commit/53bd30bda61aac3b9ee098a5f8add0b21ef87d97)) + apply the attr in DSE ([d8d842](https://github.com/llvm/llvm-project/commit/d8d8420746ad988173adf5d71e9d51cd8d3975e7)).
https://llvm-compile-time-tracker.com/compare.php?from=07d7b9c255078edc6f04bd4e68416bdf3e8735ab&to=d8d8420746ad988173adf5d71e9d51cd8d3975e7&stat=instructions%3Au



https://github.com/llvm/llvm-project/pull/84803