Implements some of the helper functions more efficiently with
lookups rathers than branches. Modeling function is consolidated
to reduce some computations.

Also merged the two enums BLOCK_SIZE_TYPES and BlockSize into
one because there is no need to keep them separate (even though
the semantics are a little different).

No bitstream or output change.

About 0.5% speedup

Change-Id: I7d71a66e8031ddb340744dc493f22976052b8f9f