fix(mir): 模2^n AND优化——修复 x<0 且 x%2^n==0 时的零值错误

-4%2 应得 0，但 AND+符号修正路径算出 -2。
增加 cmp masked,#0; csel dst,wzr,temp,EQ 零值保护。
平台无 -O 时该路径被触发，导致 48_assign_complex_expr WA。

src/mir/Lowering.cpp

@@ -606,11 +606,21 @@ namespace mir
                               Operand::VReg(masked, VRegClass::Int),
                               Operand::VReg(val_reg, VRegClass::Int)});
               }
+              int after_sign = function.CreateVReg(VRegClass::Int);
               block.Append(Opcode::Csel,
-                           {Operand::VReg(dst, VRegClass::Int),
+                           {Operand::VReg(after_sign, VRegClass::Int),
                             Operand::VReg(neg_fixup, VRegClass::Int),
                             Operand::VReg(masked, VRegClass::Int),
                             Operand::Imm(static_cast<int>(CondCode::LT))});
+              // 修正：若 masked==0 则结果必须为 0（-4 % 2 = 0，不是 -2）
+              block.Append(Opcode::CmpImm,
+                           {Operand::VReg(masked, VRegClass::Int),
+                            Operand::Imm(0)});
+              block.Append(Opcode::Csel,
+                           {Operand::VReg(dst, VRegClass::Int),
+                            Operand::Reg(PhysReg::WZR),
+                            Operand::VReg(after_sign, VRegClass::Int),
+                            Operand::Imm(static_cast<int>(CondCode::EQ))});
               value_vregs[value] = dst;
               return dst;
             }
@@ -660,11 +670,21 @@ namespace mir
                               Operand::VReg(masked, VRegClass::Int),
                               Operand::VReg(val_reg, VRegClass::Int)});
               }
+              int after_sign2 = function.CreateVReg(VRegClass::Int);
               block.Append(Opcode::Csel,
-                           {Operand::VReg(dst, VRegClass::Int),
+                           {Operand::VReg(after_sign2, VRegClass::Int),
                             Operand::VReg(neg_fixup, VRegClass::Int),
                             Operand::VReg(masked, VRegClass::Int),
                             Operand::Imm(static_cast<int>(CondCode::LT))});
+              // 修正：若 masked==0 则结果必须为 0
+              block.Append(Opcode::CmpImm,
+                           {Operand::VReg(masked, VRegClass::Int),
+                            Operand::Imm(0)});
+              block.Append(Opcode::Csel,
+                           {Operand::VReg(dst, VRegClass::Int),
+                            Operand::Reg(PhysReg::WZR),
+                            Operand::VReg(after_sign2, VRegClass::Int),
+                            Operand::Imm(static_cast<int>(CondCode::EQ))});
               value_vregs[value] = dst;
               return dst;
             }