InstCombine: Remove redundant combines

InstSimplify already handles icmp (X+Y), X (and things like it)
appropriately.  The first thing that InstCombine does is run
InstSimplify on the instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216659 91177308-0d34-0410-b5e6-96231b3b80d8