[llvm] [AMDGPU] Add dereferenceable retAttr to a call to llvm.amdgcn.implicitarg.ptr (PR #182206)

[Yoonseo Choi via llvm-commits]

yoonseoch wrote:

@arsenm, @bcahoon, I don't have privilege to merge to upstream. I updated the description at the top useable to commit messages.  

https://github.com/llvm/llvm-project/pull/182206