[Attributor] AAUndefinedBehavior: Check for branches on undef value.

A branch is considered UB if it depends on an undefined / uninitialized value.
At this point this handles simple UB branches in the form: `br i1 undef, ...`
We query `AAValueSimplify` to get a value for the branch condition, so the branch
can be more complicated than just: `br i1 undef, ...`.

Patch By: Stefanos Baziotis (@baziotis)

Reviewers: jdoerfert, sstefan1, uenoku

Reviewed By: uenoku

Differential Revision: https://reviews.llvm.org/D71799