[LV] Fix PR26600: avoid out of bounds loads for interleaved access vectorization

Summary:
If we don't have the first and last access of an interleaved load group,
the first and last wide load in the loop can do an out of bounds
access. Even though we discard results from speculative loads,
this can cause problems, since it can technically generate page faults
(or worse).

We now discard interleaved load groups that don't have the first and
load in the group.

Reviewers: hfinkel, rengolin

Subscribers: rengolin, llvm-commits, mzolotukhin, anemet

Differential Revision: http://reviews.llvm.org/D17332

llvm-svn: 261331