Merged r19549 into 3.24 branch.

Fix for failing asserts in HBoundsCheck code generation on x64: index register should be zero extended.

BUG=345820
LOG=N
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/182963002

git-svn-id: https://v8.googlecode.com/svn/branches/3.24@19572 ce2b1a6d-e550-0410-aec6-3dcde31c8c00