[NFCI][llvm-reduce] Cleanup Delta passes to use Oracle abstraction

Summary:
I think, this results in much more understandable/readable flow.
At least the original logic was perhaps the most hard thing for me to grasp when taking an initial look on the delta passes.

Reviewers: nickdesaulniers, dblaikie, diegotf, george.burgess.iv

Reviewed By: nickdesaulniers

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83287