[ConstantRange] Add an getEquivalentICmp helper

Currently only its unit test uses it, but this will be used in a later
change to simplify some logic in the GuardWidening pass.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270018 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed