Move ashr optimization from InstCombineShift to InstSimplify.
Refactor code, no functionality change, test case moved from instcombine to instsimplify.

Differential Revision: http://reviews.llvm.org/D4102
 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213231 91177308-0d34-0410-b5e6-96231b3b80d8