Eliminate useless sign extension before some subword integer operations

public static short[] addShort(short[] a, short[] b) {
        for (int i = 0; i < SIZE; i++) {
           b[i] = (short) (a[i] + 8);
           sres[i] = (short) (b[i] + 7);
         }
        return res;
    }

For the case above, SLP doesn't succeed in vectorization. But it succeeds in vectorizing similar cases of int/long/float/double, like

  public static void addInt(int[] a, int[] b) {
        for (int i = 0; i < LENGTH; i++) {
           b[i] = (a[i] + 8);
           ires[i] = (b[i] + 7);
         }
    }