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

This patch prepares ValueLatticeElement to be used by SCCP, by:
* making the mark* functions public
* make the mark* functions return a bool indicating if the value has changed.

Reviewers: efriedma, davide, mssimpso, nikic

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D60581