[clang] [llvm] [AMDGPU][Clang] Allow amdgpu-waves-per-eu attribute to lower target occupancy range (PR #138284)

[Lucas Ramirez via cfe-commits]

lucas-rami wrote:

> IIUC that is because the flat workgroup size. Waves per EU must yield to the value computed from flat workgroup size, and if it is absent, we must assume it can be 1024.

AFAIU the intent of the existing implementation is that the default minimum waves/EU is set so that all the waves of a workgroup of maximum size can fit concurrently on a single CU. I am not proposing we change that, what I would like the "amdgpu-waves-per-eu" attribute to do is be able to lower than minimum at the user's request in cases where higher occupancies are not thought to be beneficial.

https://github.com/llvm/llvm-project/pull/138284