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.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 {