InstCombine: Simplify FoldICmpCstShrCst

This function was complicated by the fact that it tried to perform
canonicalizations that were already preformed by InstSimplify.  Remove
this extra code and move the tests over to InstSimplify.  Add asserts to
make sure our preconditions hold before we make any assumptions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220314 91177308-0d34-0410-b5e6-96231b3b80d8