target/arm: Fix typo in do_sat_addsub_64

Used the wrong temporary in the computation of subtractive overflow.
Reported-by: NLaurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>
Reviewed-by: NLaurent Desnogues <laurent.desnogues@gmail.com>
Tested-by: NAlex Bennée <alex.bennee@linaro.org>
Tested-by: NLaurent Desnogues <laurent.desnogues@gmail.com>
Message-id: 20180801123111.3595-3-richard.henderson@linaro.org
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target/arm/translate-sve.c

@@ -1625,7 +1625,7 @@ static void do_sat_addsub_64(TCGv_i64 reg, TCGv_i64 val, bool u, bool d)
             /* Detect signed overflow for subtraction.  */
             tcg_gen_xor_i64(t0, reg, val);
             tcg_gen_sub_i64(t1, reg, val);
-            tcg_gen_xor_i64(reg, reg, t0);
+            tcg_gen_xor_i64(reg, reg, t1);
             tcg_gen_and_i64(t0, t0, reg);
 
             /* Bound the result.  */