[LV] Move LoopBodyTraits to a better place, and add comment for simplifying...

[LV] Move LoopBodyTraits to a better place, and add comment for simplifying LoopBlocksTraversal. NFC.

Summary: I later (after r278573) found that LoopIterator.h has some overlapping with LoopBodyTraits. It's good to use LoopBodyTraits because a *Traits struct is algorithm independent.

Reviewers: anemet, nadav, mkuper

Subscribers: mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D23529

llvm-svn: 278996