[IR] Change access rights of PredIterator members

These members were made private here https://github.com/llvm/llvm-project/commit/6177386b05ddaaee04160b153a41ed4edbd7bcaf without an explanation.
Our customers have an own implementation inherited from PredIterator with updated advancePastNonTerminators().
The access specifier protected looks resonable and safe here.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D131608