[NFCI][LoopUnrollAndJam] Minor changes.

Summary:
1. Add assertions.
2. Verify more analyses.
These changes are moved out of https://reviews.llvm.org/D73129 to
simplify that review.

Reviewer: dmgreen, jdoerfert, Meinersbur, kbarton, bmahjour, etiotto
Reviewed By: dmgreen
Subscribers: fhahn, hiraditya, zzheng, llvm-commits, prithayan, anhtuyen
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D73204