[llvm] [AMDGPU] MCExpr-ify MC layer kernel descriptor (PR #80855)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > Can you add some examples where the expression has to refer to other functions' values?
> 
> Currently there is no explicit tests I can add for this. I have one patch that converts AMDGPUResourceUsageAnalysis from a module level pass to a MachineFunction level pass and will include the infrastructure for resource usage information to be propagated through MCExprs and symbols. This propagation may mean that a symbol may be used before it is defined. However, I can't put a PR up for that unless I add support for the MC layer emit/parse for all (meta)data derived from resource usage.

I would expect the MC part to be decoupled from the codegen change to make use of it. I'm mostly wondering what the syntax ends up looking like

https://github.com/llvm/llvm-project/pull/80855