Optimize plan phase for foreground gcs (#45208)

* Changes to allow us to use the mark list for foreground GCs.

The key point is to clear the background GC mark bits for objects that the foreground GC found to be dead.

The existing code walked all the dead objects individually and cleared their mark bits, but as it turns out, it is significantly cheaper to turn off the mark bits in bulk.

* Address code review feedback - factor out common while-loop.

src/coreclr/src/gc/gc.cpp

@@ -8713,16 +8713,6 @@ void gc_heap::sort_mark_list()
         return;
     }
 
-#ifdef BACKGROUND_GC
-    // we are not going to use the mark list if background GC is running
-    // so let's not waste time sorting it
-    if (gc_heap::background_running_p())
-    {
-        mark_list_index = mark_list_end + 1;
-        return;
-    }
-#endif //BACKGROUND_GC
-
     // if any other heap had a mark list overflow, we fake one too,
     // so we don't use an incomplete mark list by mistake
     for (int i = 0; i < n_heaps; i++)
@@ -23445,11 +23435,7 @@ void gc_heap::plan_phase (int condemned_gen_number)
 #endif //GC_CONFIG_DRIVEN
 
     if ((condemned_gen_number < max_generation) &&
-        (mark_list_index <= mark_list_end)
-#ifdef BACKGROUND_GC
-        && (!gc_heap::background_running_p())
-#endif //BACKGROUND_GC
-        )
+        (mark_list_index <= mark_list_end))
     {
 #ifndef MULTIPLE_HEAPS
 #ifdef USE_VXSORT
@@ -24151,19 +24137,24 @@ void gc_heap::plan_phase (int condemned_gen_number)
 #ifdef MARK_LIST
             if (use_mark_list)
             {
-               while ((mark_list_next < mark_list_index) &&
-                      (*mark_list_next <= x))
-               {
-                   mark_list_next++;
-               }
-               if ((mark_list_next < mark_list_index)
+                uint8_t* old_x = x;
+                while ((mark_list_next < mark_list_index) &&
+                    (*mark_list_next <= x))
+                {
+                    mark_list_next++;
+                }
+                x = end;
+                if ((mark_list_next < mark_list_index)
 #ifdef MULTIPLE_HEAPS
-                   && (*mark_list_next < end) //for multiple segments
+                    && (*mark_list_next < end) //for multiple segments
 #endif //MULTIPLE_HEAPS
-                   )
-                   x = *mark_list_next;
-               else
-                   x = end;
+                    )
+                x = *mark_list_next;
+                if (current_c_gc_state == c_gc_state_marking)
+                {
+                    assert(gc_heap::background_running_p());
+                    bgc_clear_batch_mark_array_bits (old_x, x);
+                }
             }
             else
 #endif //MARK_LIST