[llvm] [AMDGPU] Compiler should synthesize private buffer resource descriptor from flat_scratch_init (PR #79586)

[via llvm-commits]

alex-t wrote:

> (Sorry for adding more nits after initial review)
> 
> I'm a bit confused about the goal; does this patch just pave the way to actually stop requesting the preloaded RSRC in some cases in the future?

Yep. It currently avoids using the preloaded RSRC if the FlatScratchInit is available as RSRC low 64 bits repeat the FlatScratchInit and the hi 64 bits are known const value. However, I still have no idea how to instruct runtime not to set s[0:3] up with the preloaded values in case the whole 128 bits are effectively synthesized in a compile time.

https://github.com/llvm/llvm-project/pull/79586