target-arm: Treat UNPREDICTABLE VTBL, VTBX case as UNDEF

Catch the UNPREDICTABLE case for Neon VTBL,VTBX, and UNDEF it
rather than allowing the helper function to index off the end
of the register file.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

target-arm/translate.c

@@ -6023,7 +6023,14 @@ static int disas_neon_data_insn(CPUState * env, DisasContext *s, uint32_t insn)
                 }
             } else if ((insn & (1 << 10)) == 0) {
                 /* VTBL, VTBX.  */
-                int n = ((insn >> 5) & 0x18) + 8;
+                int n = ((insn >> 8) & 3) + 1;
+                if ((rn + n) > 32) {
+                    /* This is UNPREDICTABLE; we choose to UNDEF to avoid the
+                     * helper function running off the end of the register file.
+                     */
+                    return 1;
+                }
+                n <<= 3;
                 if (insn & (1 << 6)) {
                     tmp = neon_load_reg(rd, 0);
                 } else {