JitArm64_Integer: subfcx - Optimize a == 0

This case can be handled as a move. It also generates a constant carry
flag.

Before:
0x52800013   mov    w19, #0x0                 ; =0
0x6b1302b3   subs   w19, w21, w19

After:
0x2a1503f3   mov    w19, w21

Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp

@@ -1432,6 +1432,17 @@ void JitArm64::subfcx(UGeckoInstruction inst)
     if (inst.Rc)
       ComputeRC0(gpr.GetImm(d));
   }
+  else if (gpr.IsImm(a, 0))
+  {
+    if (d != b)
+    {
+      gpr.BindToRegister(d, false);
+      MOV(gpr.R(d), gpr.R(b));
+    }
+    ComputeCarry(true);
+    if (inst.Rc)
+      ComputeRC0(gpr.R(d));
+  }
   else
   {
     gpr.BindToRegister(d, d == a || d == b);