Is it related to 知る？
Or the auxiliary し
Though I can't give a precise interpretation of the whole haiku, a natural reading for the part in question should be as follows:
しれし is 知れ + し, where 知れ is 連用形 (bold) of
and し is 連体形 of き, which indicates a past tense (i.e. roughly equivalent to the modern た).
Thus, literally, 桜としれし means 桜と知られた (known to be a cherry tree).