[llvm] [AMDGPU] Treat rematerialized SALU/SMEM instructions as part of the BB prolog (PR #223508)

[Gabriel Baraldi via llvm-commits]

gbaraldi wrote:

Yeah, it's somewhat of a bastardization of the flag. Though not too different from it's motivating use in https://github.com/llvm/llvm-project/pull/117544  where It was a copy and not a remat. I'm not a maintainer of the AMDGPU backend but was trying my hand with the help of the robots. 

https://github.com/llvm/llvm-project/pull/223508