InstSimplify: Improve handling of ashr/lshr

Summary:
Analyze the range of values produced by ashr/lshr cst, %V when it is
being used in an icmp.

Reviewers: nicholas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3774

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209000 91177308-0d34-0410-b5e6-96231b3b80d8