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

[Jay Foad via llvm-commits]

jayfoad wrote:

> This patch makes getScoreUB(), getScoreLB() and getScoreRange() private and introduces new functions that don't expose the internal implementation:
> 
> * getOutstanding(T)

But getOutstanding is identical to getScoreRange? Can we just have one or the other?

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