[ConstantRange] Support for ashr in ConstantRange computation

Extend the ConstantRange implementation to compute the range of possible values resulting from an arithmetic right shift operation.
There will be a follow up patch to leverage this constant range infrastructure in LazyValueInfo.

Patch by Surya Kumari Jangala!

Differential Revision: https://reviews.llvm.org/D40881

llvm-svn: 320976