Subzero: Use cvttss2si and similar instead of cvtss2si for fp->int casts.

Subzero: Use cvttss2si and similar instead of cvtss2si for fp->int casts.

This is the truncating cvt instruction instead of rounding.

A few interesting floating point inputs are added to the cross tests.

Also, the cross test error output is modified to be more clear.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/550723002