CPU/PGXP: Fix recompiled mtc0 validating its source with a garbage value

The recompiler only passes the source register, so take that alone.

src/core-tests/cpu_execution_tests.cpp

@@ -205,6 +205,30 @@ TEST_P(CPUExecution, PGXPIgnoresWritesToZero)
   CPU::CodeCache::Reset();
 }
 
+TEST_P(CPUExecution, PGXPMTC0KeepsSourceValid)
+{
+  // mtc0 validates its source register, which must not lose its flags.
+  g_settings.gpu_pgxp_enable = true;
+  g_settings.gpu_pgxp_cpu = true;
+  CPU::PGXP::Initialize();
+  CPU::CodeCache::Reset();
+  const CPU::PGXPValue zero = CPU::g_state.pgxp_gpr[0];
+
+  Program prog;
+  prog.LoadImmediate(Reg::T0, 0x12345678u);
+  prog.Emit(mtc0(Reg::T0, 3));
+  prog.Emit(mtc0(Reg::R0, 3));
+  prog.Run();
+
+  EXPECT_EQ(CPU::g_state.pgxp_gpr[static_cast<u32>(Reg::T0)].flags & CPU::PGXP::VALID_XY, CPU::PGXP::VALID_XY);
+  EXPECT_EQ(std::memcmp(&CPU::g_state.pgxp_gpr[0], &zero, sizeof(zero)), 0);
+
+  CPU::PGXP::Shutdown();
+  g_settings.gpu_pgxp_enable = false;
+  g_settings.gpu_pgxp_cpu = false;
+  CPU::CodeCache::Reset();
+}
+
 TEST_P(CPUExecution, PGXPMemoryModeMovesWithZeroImmediate)
 {
   // addiu/ori/xori with a zero immediate are moves, which memory mode PGXP keeps the value through.

src/core/cpu_core.cpp

@@ -1987,7 +1987,7 @@ restart_instruction:
             }
 
             if constexpr (pgxp_mode == PGXPMode::CPU)
-              PGXP::CPU_MTC0(inst, value, orig_value);
+              PGXP::CPU_MTC0(inst, orig_value);
           }
           break;
 

src/core/cpu_pgxp.cpp

@@ -1923,7 +1923,7 @@ void CPU::PGXP::CPU_MFC0(Instruction instr, u32 rdVal)
   prtVal.value = rdVal;
 }
 
-void CPU::PGXP::CPU_MTC0(Instruction instr, u32 rdVal, u32 rtVal)
+void CPU::PGXP::CPU_MTC0(Instruction instr, u32 rtVal)
 {
   LOG_VALUES_C1(instr.r.rt.GetValue(), rtVal);
 
@@ -1931,5 +1931,5 @@ void CPU::PGXP::CPU_MTC0(Instruction instr, u32 rdVal, u32 rtVal)
   PGXPValue& prtVal = ValidateAndGetRtValue(instr, rtVal);
   PGXPValue& prdVal = g_state.pgxp_cop0[static_cast<u8>(instr.r.rd.GetValue())];
   prdVal = prtVal;
-  prdVal.value = rdVal;
+  prdVal.value = rtVal;
 }

src/core/cpu_pgxp.h

@@ -89,7 +89,7 @@ void CPU_SLLV(Instruction instr, u32 rsVal, u32 rtVal);
 void CPU_SRLV(Instruction instr, u32 rsVal, u32 rtVal);
 void CPU_SRAV(Instruction instr, u32 rsVal, u32 rtVal);
 void CPU_MFC0(Instruction instr, u32 rdVal);
-void CPU_MTC0(Instruction instr, u32 rdVal, u32 rtVal);
+void CPU_MTC0(Instruction instr, u32 rtVal);
 
 // Utility functions.