video_core/control_flow: Prevent sign conversion in TryGetBlock()

The return value is a u32, not an s32, so this would result in an
implicit signedness conversion.

src/video_core/shader/control_flow.cpp

@@ -84,7 +84,7 @@ std::pair<BlockCollision, u32> TryGetBlock(CFGRebuildState& state, u32 address)
             return {BlockCollision::Inside, index};
         }
     }
-    return {BlockCollision::None, -1};
+    return {BlockCollision::None, 0xFFFFFFFF};
 }
 
 struct ParseInfo {