The formal parameter count was always being treated as an untagged
integer, but it is actually a Smi on ia32 and arm.

R=mstarzinger@chromium.org
BUG=v8:2355
TEST=mjsunit/harmony/generators-iteration

Review URL: https://codereview.chromium.org/17485002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15230 ce2b1a6d-e550-0410-aec6-3dcde31c8c00