Accessing Lithium operands via position is fragile and makes it impossible to
statically find all uses of a given operand. This CL is a step towards cleaning
this up, more to come...

Review URL: https://chromiumcodereview.appspot.com/10878073

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12383 ce2b1a6d-e550-0410-aec6-3dcde31c8c00