[llvm] [AMDGPU] Add intrinsic readanylane (PR #115696)

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

> It is better to describe it as "readanylane(x) is x if x is uniform, otherwise readanylane(x) is undefined".

Compared to that, `@llvm.assume(@llvm.is.uniform())` actively tells the compiler that the user _knows_ that this use of the value is uniform. But then, if the user is wrong, then the result is undefined anyway. In what way is one better than the other?

https://github.com/llvm/llvm-project/pull/115696