Some minor cleanups for efficiency

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