[SLP] Enable vectorization of GEP expressions.

The use cases look like the following:
    x->a = y->a + 10
    x->b = y->b + 12

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210342 91177308-0d34-0410-b5e6-96231b3b80d8