MacroInterpreter: Avoid left shifting negative values.

The branch target is signed, so multiply by 4 instead of left shifting by 2

src/video_core/macro_interpreter.cpp

@@ -102,11 +102,11 @@ bool MacroInterpreter::Step(const std::vector<u32>& code, bool is_delay_slot) {
         if (taken) {
             // Ignore the delay slot if the branch has the annul bit.
             if (opcode.branch_annul) {
-                pc = base_address + (opcode.immediate << 2);
+                pc = base_address + opcode.GetBranchTarget();
                 return true;
             }
 
-            delayed_pc = base_address + (opcode.immediate << 2);
+            delayed_pc = base_address + opcode.GetBranchTarget();
             // Execute one more instruction due to the delay slot.
             return Step(code, true);
         }

src/video_core/macro_interpreter.h

@@ -91,6 +91,10 @@ private:
         u32 GetBitfieldMask() const {
             return (1 << bf_size) - 1;
         }
+
+        s32 GetBranchTarget() const {
+            return static_cast<s32>(immediate * sizeof(u32));
+        }
     };
 
     union MethodAddress {