be a smi.

The inlined version works for stores to JSArrays where the key is a
smi that is within bounds of the array and the value is either
constant or a smi so we can skip the write-barrier.
Review URL: http://codereview.chromium.org/122035

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2144 ce2b1a6d-e550-0410-aec6-3dcde31c8c00