[ValueLattice] Add new state for undef constants.

This patch adds a new undef lattice state, which is used to represent
UndefValue constants or instructions producing undef.

The main difference to the unknown state is that merging undef values
with constants (or single element constant ranges) produces  the
constant/constant range, assuming all uses of the merge result will be
replaced by the found constant.

Contrary, merging non-single element ranges with undef needs to go to
overdefined. Using unknown for UndefValues currently causes mis-compiles
in CVP/LVI (PR44949) and will become problematic once we use
ValueLatticeElement for SCCP.

Reviewers: efriedma, reames, davide, nikic

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D75120