[LoopUnroll] Reorder code to max dom tree update more obvious [nfc]

This cleans up the unroll action into two phases. Phase 1 does the mechanical act of unrolling, and leaves all conditional branches in place. Phase 2 optimizes away some of the conditional branches and then simplifies the loop. The primary benefit of the reordering is that we can delete some special cases dom tree update logic.

Differential Revision: https://reviews.llvm.org/D103561