[ConstantRange] Support subtraction in makeGuaranteedNoWrapRegion.

Previously ConstantRange::makeGuaranteedNoWrapRegion only handled addition.  This adds support for subtraction.

Differential Revision: https://reviews.llvm.org/D40036

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319806 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed