Cache+ speed, reduce mallocs

Summary:
Speed of cache+ by skipping mallocs on vectors.

Although this change speeds up the algorithm by 2x, this is still not
enough for some binaries where some functions have ~2500 hot basic
blocks. Hence, introduce a threshold for expensive optimizations in
CachePlusReorderAlgorithm. If the number of hot basic blocks exceeds
the threshold (2048 by default), we use a cheaper version, which is
quite fast.

(cherry picked from FBD6928075)