[llvm] [ADT] Simplify SparseBitVectorIterator. NFCI. (PR #143885)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

> This shows that the new implementation is faster for sparse SparseBitVectors but 4x slower when they get almost full.

Do we know how full the existing `SparseBitVector`s are in LLVM on average?

https://github.com/llvm/llvm-project/pull/143885