ARM: Add comments to clarify conditionally executed code dealing with d16-d31

Review URL: https://codereview.chromium.org/12087132
Patch from Hans Wennborg <hans@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13578 ce2b1a6d-e550-0410-aec6-3dcde31c8c00