Assert that we don't RAUW a Constant with a ConstantExpr that contains it.

We already had an assert for foo->RAUW(foo), but not for something like
foo->RAUW(GEP(foo)) and would go in an infinite loop trying to apply
the replacement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208663 91177308-0d34-0410-b5e6-96231b3b80d8