Subzero: x8664: Fix a "uitofp i32 to double" lowering error.

The movzx instruction in the lowering sequence should not be allowed to be elided.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4379
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/2335933002 .
1 file changed