Fixup test to check for GCN prefix
These patterns always zero extend the result even though it might need sign extension.
This has been broken since the addition of i16 support.
It has popped up in mad_sat(char) test since min(max()) combination is turned into v_med3, resulting in the following (incorrect) sequence:
v_mad_i16 v2, v10, v9, v11 v_med3_i32 v2, v2, v8, v7
Fixes mad_sat(char) piglit on VI.