[Analysis] In EmitGEPOffset, use Constant::getUniqueInteger to handle struct...

[Analysis] In EmitGEPOffset, use Constant::getUniqueInteger to handle struct indices in vector GEPs.

We previously called getSplatValue if the index had a vector type,
but getSplatValue returns null for non-splats. This would cause
a nullptr dereference if it wasn't a splat.

Using getUniqueInteger gives us an assert if its a vector type,
but the value isn't a splat. This is what is used in
SelectionDAGBuilder's code that expands GEPs as well.

llvm-svn: 370001