[ValueLattice] Update markConstantRange to return false equal ranges.

Currently we always return true, when markConstantRange is used on an
object already containing a constant range. If NewR is equal to the
existing constant range however, nothing changes and we should return
false.

I also went ahead and added a clarifying comment and improved the
assertion.

Reviewers: efriedma, davide, nikic

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D73240