[llvm] [BasicTTIImpl] Add cost entries for ldexp, [l]lround (PR #146373)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Did @arsenm hit "approved" by accident? AMDGPU costs are completely wrong. I'm unfamiliar with AMDGPU, so @arsenm: can you handle it in a pre-commit or follow-up?

Not an accident. The AMDGPU costs being wrong have little to do with this patch 

https://github.com/llvm/llvm-project/pull/146373