[GlobalDCE] In VFE support for relative pointers, allow GEP references to the base symbol

This is for Swift VFE support. In some vtable forms that Swift emits, the "base" of a relative pointer is not the global symbol itself directly, but a GEP into it -- so the pointer is relative to a particular field in the global. So getPointerAtOffset() needs to be able to see through the GEP and allow it in a SUB expression, to correctly recognize the offset as a vtable slot.

Differential Revision: https://reviews.llvm.org/D109169