[TTI] Add a hook for specifying per-target defaults for Interleaved Accesses

[TTI] Add a hook for specifying per-target defaults for Interleaved Accesses

Summary:
This adds a hook to TTI which enables us to selectively turn on by default
interleaved access vectorization for targets on which we have have performed
the required benchmarking.

Reviewers: rengolin

Subscribers: rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D11901

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244449 91177308-0d34-0410-b5e6-96231b3b80d8