[llvm] [AMDGPU][NFC] Use `getScoreUB` for XCNT insertion. (PR #162448)

[Christudasan Devadasan via llvm-commits]

cdevadas wrote:

> I'm not sure if we need a test here?

This is a better way of achieving the code change that was introduced in https://github.com/llvm/llvm-project/pull/160812.

https://github.com/llvm/llvm-project/pull/162448