[LoopFlatten] FlattenInfo bookkeeping. NFC.

Introduce struct FlattenInfo to group some of the bookkeeping. Besides this
being a bit of a clean-up, it is a prep step for next additions (D90640). I
could take things a bit further, but thought this was a good first step also
not to make this change too large.

Differential Revision: https://reviews.llvm.org/D90408