Move the value of the root-array register to offset 128 from the start of
the root array. This allows indices 16..31 to be reached using only an
8-bit displacement, saving three bytes per access.

Review URL: http://codereview.chromium.org/6594115

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7069 ce2b1a6d-e550-0410-aec6-3dcde31c8c00