Use lowerCast instead of inlined _movzx, to get legalization, for memset.

Otherwise, there can be a movzx reg, 0, which is illegal,
when the memset value is constant 0.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3882
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/402253002
2 files changed