Revert 209903 and 210040.

The messages were

 "PR19753: Optimize comparisons with "ashr exact" of a constanst."
 "Added support to optimize comparisons with "lshr exact" of a constant."

They were not correctly handling signed/unsigned operation differences,
causing pr19958.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210393 91177308-0d34-0410-b5e6-96231b3b80d8