Exclude <input> and other input tags from being placeholders

[mkhatib]

Now that we plan to support forms #3343 we need to make sure AMP differentiate between the AMP placeholder attribute to indicate an element is a placeholder for its parent and the placeholder="some text" on input-types to show a placeholder text inside the input field.

One way to do this is by just excluding inputs from being AMP placeholders. It doesn't actually make sense since AMP placeholders are mostly used as a waiting to be loaded so I am here until then.