Replace BuildFastElementAccess() with AddFastElementAccess().

This is trivial cleanup. All calls to BuildFastElementAccess() pass
the result to AddInstruction().

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/19759003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15789 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Add AssertSmi() in x64 MacroAssembler::SmiTest().

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/19647006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15790 ce2b1a6d-e550-0410-aec6-3dcde31c8c00