[LAA] LLE 4/6: APIs to access the dependent instructions for a dependence, NFC

Summary:
The functions use LAI and MemoryDepChecker classes so they need to be
defined after those definitions outside of the Dependence class.

Will be used by the LoopLoadElimination pass.

Reviewers: hfinkel

Subscribers: rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D13257

llvm-svn: 252015