With this CL we clearly distinguish two different views on Lithium
instructions: For register allocation, the actual instruction/operand
is irrelevant, so it has only an iterator/indexed view on the
instruction operands. All other places, most importantly code
generation, use named getters for the operands now, making it easy to
see where each one is used.

Review URL: https://codereview.chromium.org/10919261

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12510 ce2b1a6d-e550-0410-aec6-3dcde31c8c00