Rename remove_surplus_regions->trim_region_list and add_regions->grow_region_list.

Also rename and reorder parameters to dest/src and drop references to "surplus"
because these are general utilities.  Drop a few comments that aren't adding
anything beyond the code.

Author: markples

src/coreclr/gc/gc.cpp

@@ -12798,35 +12798,29 @@ void gc_heap::rearrange_heap_segments(BOOL compacting)
 #endif //!USE_REGIONS
 
 #if defined(USE_REGIONS)
-// trim down the list of free regions pointed at by free_list down to target_count, moving the extra ones to surplus_list
-static void remove_surplus_regions (region_free_list* free_list, region_free_list* surplus_list, size_t target_count)
+// trim down the list of regions pointed at by src down to target_count, moving the extra ones to dest
+static void trim_region_list (region_free_list* dest, region_free_list* src, size_t target_count)
 {
-    while (free_list->get_num_free_regions() > target_count)
+    while (src->get_num_free_regions() > target_count)
     {
-        // remove one region from the heap's free list
-        heap_segment* region = free_list->unlink_region_front();
-
-        // and put it on the surplus list
-        surplus_list->add_region_front (region);
+        heap_segment* region = src->unlink_region_front();
+        dest->add_region_front (region);
     }
 }
 
-// add regions from surplus_list to free_list, trying to reach target_count
-static int64_t add_regions (region_free_list* free_list, region_free_list* surplus_list, size_t target_count)
+// add regions from src to dest, trying to grow the size of dest to target_count
+static int64_t grow_region_list (region_free_list* dest, region_free_list* src, size_t target_count)
 {
     int64_t added_count = 0;
-    while (free_list->get_num_free_regions() < target_count)
+    while (dest->get_num_free_regions() < target_count)
     {
-        if (surplus_list->get_num_free_regions() == 0)
+        if (src->get_num_free_regions() == 0)
             break;
 
         added_count++;
 
-        // remove one region from the surplus list
-        heap_segment* region = surplus_list->unlink_region_front();
-
-        // and put it on the heap's free list
-        free_list->add_region_front (region);
+        heap_segment* region = src->unlink_region_front();
+        dest->add_region_front (region);
     }
     return added_count;
 }
@@ -13582,7 +13576,7 @@ void gc_heap::distribute_free_regions()
                     hp->free_regions[kind].get_num_free_regions(),
                     heap_budget_in_region_units[kind][i]));
 
-                remove_surplus_regions (&hp->free_regions[kind], &surplus_regions[kind], heap_budget_in_region_units[kind][i]);
+                trim_region_list (&surplus_regions[kind], &hp->free_regions[kind], heap_budget_in_region_units[kind][i]);
             }
         }
         // finally go through all the heaps and distribute any surplus regions to heaps having too few free regions
@@ -13598,7 +13592,7 @@ void gc_heap::distribute_free_regions()
             // second pass: fill all the regions having less than budget
             if (hp->free_regions[kind].get_num_free_regions() < heap_budget_in_region_units[kind][i])
             {
-                int64_t num_added_regions = add_regions (&hp->free_regions[kind], &surplus_regions[kind], heap_budget_in_region_units[kind][i]);
+                int64_t num_added_regions = grow_region_list (&hp->free_regions[kind], &surplus_regions[kind], heap_budget_in_region_units[kind][i]);
                 dprintf (REGIONS_LOG, ("added %zd %s regions to heap %d - now has %zd, budget is %zd",
                     (size_t)num_added_regions,
                     free_region_kind_name[kind],