Split AC-Tile MPM context into Search and Initialization structures.

Some of the fields in the SCACTileCtx struct are only used to create the MPM,
but are not needed to search the MPM. Create a new structure to contain just
the data needed by AC Search. After creating the MPM, copy the data into the
new structure and then free the memory only needed during initialization.

This reduces the size of the AC-Tile MPM context from 1360 bytes down to 296
bytes.