blockfreq: Use getSuccessorIndex()

No functionality change.

<rdar://problem/14292693>
blockfreq: Rename BlockFrequencyImpl to BlockFrequencyInfoImpl

This is a shared implementation class for BlockFrequencyInfo and
MachineBlockFrequencyInfo, not for BlockFrequency, a related (but
distinct) class.

No functionality change.

<rdar://problem/14292693>
blockfreq: Remove unnecessary template parameters

Moves redundant template parameters into an implementation detail of
BlockFrequencyInfoImpl.

No functionality change.

<rdar://problem/14292693>
blockfreq: Rename BlockProbInfoT to BranchProbabilityInfoT

No functionality change.

<rdar://problem/14292693>
blockfreq: Document BlockFrequencyInfo terminology

Documents terminology used in the forthcoming rewrite of
BlockFrequencyInfo.

<rdar://problem/14292693>