x64: Make sure that the upper half of a 64bit register contains 0 for int32 values.

BUG=360611
LOG=y
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/225393005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20664 ce2b1a6d-e550-0410-aec6-3dcde31c8c00