tcg: Initialize return value after exit_atomic

Users of tcg_gen_atomic_cmpxchg and do_atomic_op rightfully utilize
the output.  Even though this code is dead, it gets translated, and
without the initialization we encounter a tcg_error.

Reported-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Tested-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

tcg/tcg-op.c

@@ -2861,6 +2861,9 @@ void tcg_gen_atomic_cmpxchg_i64(TCGv_i64 retv, TCGv addr, TCGv_i64 cmpv,
 #endif
 #else
         gen_helper_exit_atomic(tcg_ctx.tcg_env);
+        /* Produce a result, so that we have a well-formed opcode stream
+           with respect to uses of the result in the (dead) code following.  */
+        tcg_gen_movi_i64(retv, 0);
 #endif /* CONFIG_ATOMIC64 */
     } else {
         TCGv_i32 c32 = tcg_temp_new_i32();
@@ -2966,6 +2969,9 @@ static void do_atomic_op_i64(TCGv_i64 ret, TCGv addr, TCGv_i64 val,
 #endif
 #else
         gen_helper_exit_atomic(tcg_ctx.tcg_env);
+        /* Produce a result, so that we have a well-formed opcode stream
+           with respect to uses of the result in the (dead) code following.  */
+        tcg_gen_movi_i64(ret, 0);
 #endif /* CONFIG_ATOMIC64 */
     } else {
         TCGv_i32 v32 = tcg_temp_new_i32();