Implements moves between double and i64. It also completes the
implementation of the IR "move" instruction, except for vectors.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334
R=eholk@chromium.org, jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1642253002 .