x64: save a bit code size

It would be better use macro always

BUG=
R=verwaest@chromium.org

Review URL: https://chromiumcodereview.appspot.com/100843002

Patch from Weiliang Lin <weiliang.lin@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18282 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
HLoadNamedField for Smis optimized for x64

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/108633003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18283 ce2b1a6d-e550-0410-aec6-3dcde31c8c00