[Attributor] Function level undefined behavior attribute

_Eventually_, this attribute will be assigned to a function if it
contains undefined behavior. As a first small step, I tried to make it
loop through the load instructions in a function (eventually, the plan
is to check if a load instructions causes undefined behavior, because
e.g. dereferences a null pointer - Also eventually, this won't happen in
initialize() but in updateImpl()).

Patch By: Stefanos Baziotis (@baziotis)

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D71435