Restrict BCJ decoding to just the ones in use in the encoder.

And, more importantly, explain why. We never wrote an encoder for
arm64, so that one can safely be removed. The 32-bit ones need to
stay, but that does mean that when we drop 32-bit support, they can
go too.

Change-Id: I6031fd6cb7656eecc3554a80e27a294608028699
1 file changed