Handle branch delay slots

* MIPS has an architectural feature, where instead of flushing the
pipeline when executing a branch instruction, it goes ahead and executes
the instruction following the branch as well. Flushing the pipeline
is costly and is the cause of those complex branch predictors on
modern CPUs that try to guess when a branch will be taken.

* To properly emulate this behaviour we must act how the pipeline acts. We
will always have 2 instructions loaded the current one and the next one. This way
we can load the instruction after the branch even though the pc changes.

include/cpu/ee.hpp

@@ -63,6 +63,7 @@ public:
     /* Opcodes */
     void op_cop0(); void op_mfc0(); void op_sw();
     void op_special(); void op_sll(); void op_slti();
+    void op_bne();
 
 protected:
     ComponentManager* manager;
@@ -70,9 +71,9 @@ protected:
     /* Registers. */
     Register gpr[32];
     uint32_t pc;
-    uint64_t hi0, hi1, lo0, lo1; /* Used to store multiplication and division results */
-    uint32_t sa; /* Specifies the shift amount used by the funnel shift instruction */
-    Instruction instr;
+    uint64_t hi0, hi1, lo0, lo1;
+    uint32_t sa;
+    Instruction instr, next_instr;
 
     /* Coprocessors */
     COP0 cop0;

src/cpu/ee.cpp

@@ -21,25 +21,31 @@ void EmotionEngine::reset_state()
 {
     /* Reset PC. */
     pc = 0xbfc00000;
+
+    /* Reset instruction holders */
+    instr.value = 0;
+    next_instr.value = 0;
 }
 
 void EmotionEngine::fetch_instruction()
 {
-    instr.value = manager->read_memory<uint32_t>(pc);
+    /* Handle branch delay slots by prefetching the next one */
+    instr.value = next_instr.value;
+    next_instr = manager->read_memory<uint32_t>(pc);
+    pc += 4;
+    
     std::cout << "PC: " << std::hex << pc << " Instruction: " << instr.value << '\n';
-
     switch(instr.opcode)
     {
     case COP0_OPCODE: op_cop0(); break;
     case SPECIAL_OPCODE: op_special(); break;
     case 0b101011: op_sw(); break;
     case 0b001010: op_slti(); break;
+    case 0b000101: op_bne(); break;
     default:
         std::cout << "Unimplemented opcode: " << std::bitset<6>(instr.opcode) << '\n';
         std::abort();
     }
-
-    pc += 4;
 }
 
 void EmotionEngine::op_cop0()