[TTI] Fix default costs for interleaved accesses

Summary:
Modify the cost calculation function for interleaved accesses
to use the target-specific costs for insert/extract element and
memory operations.

This better models the case where the backend can't match
the interleaved group, and we are forced to use a wide load
and shuffle vectors.

Interleaved accesses are not enabled by default, so this shouldn't
cause a performance change.

Reviewers: jmolloy

Subscribers: jmolloy, llvm-commits

Differential Revision: http://reviews.llvm.org/D11718

llvm-svn: 243875