target/arm: Convert Neon narrowing shifts with op==8 to decodetree

Convert the Neon narrowing shifts where op==8 to decodetree:
 * VSHRN
 * VRSHRN
 * VQSHRUN
 * VQRSHRUN

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200522145520.6778-6-peter.maydell@linaro.org

target/arm/translate.c

@@ -5280,6 +5280,7 @@ static int disas_neon_data_insn(DisasContext *s, uint32_t insn)
             case 5: /* VSHL, VSLI */
             case 6: /* VQSHLU */
             case 7: /* VQSHL */
+            case 8: /* VSHRN, VRSHRN, VQSHRUN, VQRSHRUN */
                 return 1; /* handled by decodetree */
             default:
                 break;