ConstantExpr, not just the top-level operator. This allows it to
fold many more constants.
Also, make GlobalOpt call ConstantFoldConstantExpression on
GlobalVariable initializers.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89659 91177308-0d34-0410-b5e6-96231b3b80d8