vector: Split placeholder rules in search.less (again)

These rules may not be merged because of how CSS requires browsers to
parse unrecognized selectors – if any part of the selector is unknown,
the entire rule will be dropped.

Bug: 60716
Change-Id: Ief9a5a178058eb01ab032cfae8639b0fc52c47a4
Merge "vector: Split placeholder rules in search.less (again)"