Neon: Update mbfilter if all vectors follow one branch.

Change the mbfilter Neon code from executing both branches if all
vectors follow only one branch.

The code is about 5% faster when executing only one branch and about
1% slower when executing both branches.

-PS5: Remove local stack space from mbfilter.

Change-Id: I6a23f9b318a9f4568a2718b4c9348db988fe2182