PointerBrightening: GEP flattening of structure types

[carsonharmon]

There were some issues with flattening GEP's for structs, vector and array types. I think it was a straightforward issue of checking the type in some other visitor functions, but right now its not immediately needed. We can remove the checks in flattenGEP when we handle the cases. It's been awhile, but I think a tests in the suite which have memcpy trigger the issue.