[llvm] [AMDGPU] Handle memcpy()-like ops in LowerBufferFatPointers (PR #126621)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > runs before codegenprepare, which normally handles unsupported memcpy
> 
> Not codegenprepare's job. Do you mean PreIselIntrinsicLowering?

Plus not all the cases get expanded there 

https://github.com/llvm/llvm-project/pull/126621