[LoopFusion] Restrict loop fusion to rotated loops.

Summary:
This patch restricts loop fusion to only consider rotated loops as valid candidates.
This simplifies the analysis and transformation and aligns with other loop optimizations.

Reviewers: jdoerfert, Meinersbur, dmgreen, etiotto, Whitney, fhahn, hfinkel

Reviewed By: Meinersbur

Subscribers: ormris, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71025