shader_recompiler, video_core: Resolve clang errors

Silences the following warnings-turned-errors:
-Wsign-conversion
-Wunused-private-field
-Wbraced-scalar-init
-Wunused-variable

And some other errors

src/shader_recompiler/frontend/maxwell/control_flow.h

@@ -161,7 +161,6 @@ private:
     Environment& env;
     ObjectPool<Block>& block_pool;
     boost::container::small_vector<Function, 1> functions;
-    FunctionId current_function_id{0};
     Location program_start;
     bool exits_to_dispatcher{};
     Block* dispatch_block{};