tcg/aarch64: Fix constant subtraction in tcg_out_addsub2

An hppa guest executing

0x000000000000e05c:  ldil L%10000,r4
0x000000000000e060:  ldo 0(r4),r4
0x000000000000e064:  sub r3,r4,sp

produces

 ---- 000000000000e064 000000000000e068
 sub2_i32 tmp0,tmp4,r3,$0x1,$0x10000,$0x0

after folding and constant propagation.  Then we hit

tcg-target.c.inc:640: tcg_out_insn_3401: Assertion `aimm <= 0xfff' failed.

because aimm is in fact -16, but unsigned.

The ((bl < 0) ^ sub) condition which negates bl is incorrect and will
always lead to this abort.  If the constant is positive, sub will make
it negative; if the constant is negative, sub will keep it negative.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tcg/aarch64/tcg-target.c.inc

@@ -1410,10 +1410,10 @@ static void tcg_out_addsubi(TCGContext *s, int ext, TCGReg rd,
     }
 }
 
-static inline void tcg_out_addsub2(TCGContext *s, TCGType ext, TCGReg rl,
-                                   TCGReg rh, TCGReg al, TCGReg ah,
-                                   tcg_target_long bl, tcg_target_long bh,
-                                   bool const_bl, bool const_bh, bool sub)
+static void tcg_out_addsub2(TCGContext *s, TCGType ext, TCGReg rl,
+                            TCGReg rh, TCGReg al, TCGReg ah,
+                            tcg_target_long bl, tcg_target_long bh,
+                            bool const_bl, bool const_bh, bool sub)
 {
     TCGReg orig_rl = rl;
     AArch64Insn insn;
@@ -1423,11 +1423,13 @@ static inline void tcg_out_addsub2(TCGContext *s, TCGType ext, TCGReg rl,
     }
 
     if (const_bl) {
-        insn = I3401_ADDSI;
-        if ((bl < 0) ^ sub) {
-            insn = I3401_SUBSI;
+        if (bl < 0) {
             bl = -bl;
+            insn = sub ? I3401_ADDSI : I3401_SUBSI;
+        } else {
+            insn = sub ? I3401_SUBSI : I3401_ADDSI;
         }
+
         if (unlikely(al == TCG_REG_XZR)) {
             /* ??? We want to allow al to be zero for the benefit of
                negation via subtraction.  However, that leaves open the