A64: Fix some int32 accesses in lithium

This fixes mjsunit/sin-cos. There are further int32 accesses being investigated.

BUG=
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/163553005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19358 ce2b1a6d-e550-0410-aec6-3dcde31c8c00