arm64: Oops, fix flushing zero from an armreg.

Core/MIPS/ARM64/Arm64CompALU.cpp

@@ -208,6 +208,7 @@ void Arm64Jit::CompType3(MIPSGPReg rd, MIPSGPReg rs, MIPSGPReg rt, void (ARM64XE
 			return;
 		}
 		// If rd is rhs, we may have lost it in the MapDirtyIn().  lhs was kept.
+		// This means the rhsImm value was never flushed to rhs, and would be garbage.
 		if (rd == rhs) {
 			// Luckily, it was just an imm.
 			gpr.SetImm(rhs, rhsImm);