Added MACRO for reference frame encoding

Added MACRO for reference frame encoding

This CL introduces a few macros plus code cleaning on the encoding of
the reference frames. Coding performance remains unchanged.

For the encoding of either the compound reference or the single reference
case, since each bit has different contexts, the tree structure may not
be applied to treat the combined bits as one symbol. It is possible we may
explore the sharing of the same context for all the bits to introduce
the use of tree structure for the next step.

Change-Id: I6916ae53c66be1a0b23e6273811c0139515484df