Split -vendor-placeholder selector by vendor.

Some browsers (specifically Firefox(18) that I know of) fail to select
correctly if the selectors are combined. This was fixed in bootstrap
(https://github.com/twitter/bootstrap/blob/master/less/forms.less#L210).
Not sure of the exact commit but there is a nice comment.

This is related to http://meta.discourse.org/t/compose-forms-inline-labels-not-properly-ghosted/1326

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