MB-8571: Optimize vtree node decoding

When compiling with the +bin_opt_info option, you get hints
where to optimize the code. The solution was to match the
binary in the function and not the function body.

Change-Id: I244e5399a5ef226f05b0f163ffa1e3f3777baf82
Reviewed-on: http://review.couchbase.org/27568
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: buildbot <build@couchbase.com>