LoopVectorize: Look for consecutive acces in GEPs with trailing zero indices

If we have a pointer to a single-element struct we can still build wide loads
and stores to it (if there is no padding).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193860 91177308-0d34-0410-b5e6-96231b3b80d8