[llvm] [IR] Update ValueHandle PrevPtr via move constructor (NFC) (PR #227484)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Since this patch makes the callback `erase` and friend class `ValueHandleBase` in DenseMap.h dead, could you fold that removal in here? The patch's justification is removing that ADT special case, so it reads better as one change, and a callerless private template won't trigger any unused warning if the follow-up slips.

https://github.com/llvm/llvm-project/pull/227484