[llvm] [AMDGPU] Allow 0 as min./max. of amdgpu-waves-per-eu (PR #138284)

[Shilei Tian via llvm-commits]

shiltian wrote:

Can we instead disallow using just one value? I find it super annoying to have something like `std::optional<int, std::optional<int>>` in the parser code.

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