Backport 4878aa36 [ValueLattice] Add new state for undef constants.

Summary:
NOTE: I mostly put it on Phabricator to make it easy for other people to fetch & check if that fixes a bug.

This patch backports 4878aa36 and required earlier patches onto
the release/10.x branch.

It includes the following patches:

aa5ebfdf [ValueLattice] Make mark* functions public, return if value changed.

c1943b42 [ValueLattice] Update markConstantRange to return false equal ranges.

e30c2578 [CVP,SCCP] Precommit test for D75055.

4878aa36 [ValueLattice] Add new state for undef constants.

All patches except the last one apply cleanly. For the last one, the
changes to SCCP.cpp were stripped, because SCCP does not yet use
ValueLattice on release/10.x. Otherwise we would have to pull in more
additional changes.

Subscribers: tstellar, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76596