[llvm] [AMDGPU] Move LowerBufferFatPointers after LoadStoreVectorizer and remove the fixme (PR #161531)

[Nicolai Hähnle via llvm-commits]

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

I took the liberty of changing the title capitalization.

This seems pretty reasonable to me.

https://github.com/llvm/llvm-project/pull/161531