A64: Make the Decoder a template

This will allow for replacing the dispatcher with a concrete decoder
visitor.

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

Review URL: https://codereview.chromium.org/181253002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19562 ce2b1a6d-e550-0410-aec6-3dcde31c8c00