[LoopUnroll] Eliminate PreserveOnlyFirst parameter [nfc]

This is a first step towards simplifying the transform interface to be less error prone. The basic idea is that querying SCEV is cheap (since it's cached) and we can just check for properties related to branch folding in the transform method instead of relying on the heuristic part to pass everything in correctly.

Differential Revision: https://reviews.llvm.org/D103584