Teach CorrelatedValuePropagation to mark adds as no wrap

This is a resubmission of previously reverted r277592. It was hitting overly strong assertion in getConstantRange which was relaxed in r278217.

Use LVI to prove that adds do not wrap. The change is motivated by https://llvm.org/bugs/show_bug.cgi?id=28620 bug and it's the first step to fix that problem.

Reviewed By: sanjoy

Differential Revision: http://reviews.llvm.org/D23059

llvm-svn: 278220