[IR] Don't accept nullptr as GEP element type

GetElementPtrInst::Create() (and IRBuilder methods based on it)
currently accept nullptr as the element type, and will fetch the
element type from the pointer in that case. Remove this fallback,
as it is incompatible with opaque pointers. I've removed a handful
of leftover calls using this behavior as a preliminary step.

Out-of-tree code affected by this change should either pass a proper
type, or can temporarily explicitly call getPointerElementType(),
if the newly added assertion is encountered.

Differential Revision: https://reviews.llvm.org/D105653