[LV] Allow interleaved accesses in loops with predicated blocks

This patch allows the formation of interleaved access groups in loops
containing predicated blocks. However, the predicated accesses are prevented
from forming groups.

Differential Revision: https://reviews.llvm.org/D19694

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275471 91177308-0d34-0410-b5e6-96231b3b80d8