[BOLT] faster cache+ implementation

Summary:
Speeding up cache+ algorithm.

The idea is to find and merge "fallthrough" successors before main
optimization. For a pair of blocks, A and B, block B is the fallthrough
successor of A, if (i) all jumps (based on profile) from A goes to B
and (ii) all jumps to B are from A.
Such blocks should be adjacent in an optimal ordering, and should
not be considered for splitting. (This gives the speed up).

The gap between cache and cache+ reduced from ~2m to ~1m.

(cherry picked from FBD6799900)