[Delinearization] Refactoring of fixed-size array delinearization

This is a follow-up patch to D122857 where we added delinearization of
fixed-size arrays to loop cache analysis, which resulted in some duplicate
code, i.e., "tryDelinearizeFixedSize()", in LoopCacheCost.cpp and
DependenceAnalysis.cpp. Refactoring is done in this patch.

This patch refactors out the main logic of "tryDelinearizeFixedSize()" as
"tryDelinearizeFixedSizeImpl()" and moves it to Delinearization.cpp, such that
clients can reuse "llvm::tryDelinearizeFixedSizeImpl()" wherever they would
like to delinearize fixed-size arrays. Currently it has two users, i.e.,
DependenceAnalysis.cpp and LoopCacheCost.cpp.

Reviewed By: Meinersbur, #loopoptwg

Differential Revision: https://reviews.llvm.org/D124745