Revert "InstCombine: merge constants in both operands of icmp."

This reverts commit r204912, and follow-up commit r204948.

This introduced a performance regression, and the fix is not completely
clear yet.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205010 91177308-0d34-0410-b5e6-96231b3b80d8