[ConstantFolding] Check return value of ConstantFoldInstOperandsImpl()

This operation is fallible, but ConstantFoldConstantImpl() is not.
If we fail to fold, we should simply return the original expression.

I don't think this can cause any issues right now, but it becomes
a problem if once make ConstantFoldInstOperandsImpl() not create a
constant expression for everything it possibly could.