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

[Arthur Eubanks via llvm-commits]

https://github.com/aeubanks approved this pull request.

lgtm with one last nit

I'd like some other approvals before landing this

for the followup patches, the inference patch generally lgtm (since I wrote a lot of it :) ), and the DSE patch in principle makes sense (the important thing is that it allows an instruction to be associated with multiple MemoryDefs), although I still need to understand it better. 0.07% compile time regression for interprocedural DSE seems acceptable IMO

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