[llvm] [AMDGPU][SIInsertWaitcnts][NFC] Make a few WaitcntBracket member functions private (PR #180018)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `lldb-aarch64-windows` running on `linaro-armv8-windows-msvc-05` while building `llvm` at step 6 "test".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/141/builds/15161

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test) failure: build (failure)

```

</details>

https://github.com/llvm/llvm-project/pull/180018