Introduce ReadPositionFromRegister in x64 Regexp Macro Assembler

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/220453008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20421 ce2b1a6d-e550-0410-aec6-3dcde31c8c00