To fully support hydrogen code stubs which accept a variable number of arguments,
the HReturn/LReturn instruction needs to be able to determine argument count
from a stack evaluation rather than as a constant from scope.

R=danno@chromium.org
BUG=

Review URL: https://codereview.chromium.org/12613004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13888 ce2b1a6d-e550-0410-aec6-3dcde31c8c00