[SLPVectorizer] Handle GEP with differing constant index types

Summary:
This fixes PR27617.

Bug description: The SLPVectorizer asserts on encountering GEPs with different index types, such as i8 and i64.

The patch includes a simple relaxation of the assert to allow constants being of different types, along with a regression test that will provoke the unrelaxed assert.

Reviewers: nadav, mzolotukhin

Subscribers: JesperAntonsson, llvm-commits, mzolotukhin

Differential Revision: http://reviews.llvm.org/D20685

Patch by Jesper Antonsson!

llvm-svn: 272206