[ConstantRange] Add getNonEmpty() constructor

ConstantRanges have an annoying special case: If upper and lower are
the same, it can be either an empty or a full set. When constructing
constant ranges nearly always a full set is intended, but this still
requires an explicit check in many places.

This revision adds a getNonEmpty() constructor that disambiguates this
case: If upper and lower are the same, a full set is created.

Differential Revision: https://reviews.llvm.org/D60947

llvm-svn: 358854