* The 'compare and swap' operations should enforce memory ordering even when
  the exchange does not occur.

* The exclusive monitor does not need to be cleared by CLREX if a LDRX was
  not followed by a matching STREX.

* Use LDAR and STLR where possible.

* Use the 'I' and 'J' constraints to hint for constants valid for immediate
  values.

R=jfb@chromium.org, rmcilroy@chromium.org, ulan@chromium.org

Review URL: https://codereview.chromium.org/220793002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20446 ce2b1a6d-e550-0410-aec6-3dcde31c8c00