Robustified address calculations on A64.

We no longer rely on the (adventurous) assumption that the class
Instruction is empty, implying sizeof(Instruction) == 1. This will
greatly simplify upcoming refactorings.

R=rodolph.perfetta@gmail.com

Review URL: https://codereview.chromium.org/201843003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20018 ce2b1a6d-e550-0410-aec6-3dcde31c8c00