[x64] 64-bit shift left by 32 or more covers sign/zero extend.

TEST=unittests
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/704703002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25123 ce2b1a6d-e550-0410-aec6-3dcde31c8c00