[Unroll] Extract the logic for caching SCEV-modeled GEPs with their

[Unroll] Extract the logic for caching SCEV-modeled GEPs with their
simplified model for use simulating each iteration into a separate
helper function that just returns the cache.

Building this cache had nothing to do with the rest of the unroll
analysis and so this removes an unnecessary coupling, etc. It should
also make it easier to think about the concept of providing fast cached
access to basic SCEV models as an orthogonal concept to the overall
unroll simulation.

I'd really like to see this kind of caching logic folded into SCEV
itself, it seems weird for us to provide it at this layer rather than
making repeated queries into SCEV fast all on their own.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237993 91177308-0d34-0410-b5e6-96231b3b80d8