BlockBasedTable::PrefixMayMatch() to bloom setting to the beginning of the function

Summary: In BlockBasedTable::PrefixMayMatch() we calculate prefix even if bloom is not config. Move the check before

Test Plan: make all check

Reviewers: igor, ljin

Reviewed By: ljin

Subscribers: wuj, leveldb, haobo, yhchiang, dhruba

Differential Revision: https://reviews.facebook.net/D18993