[LoopPeel] Expose ValueMap of last peeled iteration. NFC

The value map of last peeled iteration is computed within peelLoop API.
This patch exposes it for callers of peelLoop.
While this is not currently used by upstream passes, we have a usecase
downstream which benefits from this API update. Future users of peelLoop
can also use the ValueMap if needed.

Similar value maps are exposed by other loop utilities such as loop
cloning.

Differential Revision: https://reviews.llvm.org/D138228