[llvm] [AMDGPU] Support divergent sized dynamic alloca (PR #121148)

[via llvm-commits]

easyonaadit wrote:

> > Currently, AMDGPU backend supports static sized allocas.
> > This patch extends support for truly dynamic sized allocas.
> 
> Update this description as well as the title.

Done. I have tried to explicitly mention what is not supported, hopefully its no longer ambiguous. 

https://github.com/llvm/llvm-project/pull/121148