A64: Move the dispatching logic of the decoder to a separate class.

BUG=none
R=ulan@chromium.org, rodolph.perfetta@arm.com
LOG=n

Review URL: https://codereview.chromium.org/181233002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19561 ce2b1a6d-e550-0410-aec6-3dcde31c8c00