Cleanup: Use `switch` and `BLI_assert_unreachable()` more.

Replace some `if/else if` chains by proper `switch` statement.

Replace some `BLI_assert(0)` calls by `BLI_assert_unreachable()` ones.

source/blender/blenloader/intern/readfile.c

@@ -2413,7 +2413,7 @@ static int lib_link_main_data_restore_cb(LibraryIDLinkCallbackData *cb_data)
     if (collection->flag & COLLECTION_IS_MASTER) {
       /* We should never reach that point anymore, since master collection private ID should be
        * properly tagged with IDWALK_CB_EMBEDDED. */
-      BLI_assert(0);
+      BLI_assert_unreachable();
       return IDWALK_RET_NOP;
     }
   }

source/blender/blenloader/intern/undofile.c

@@ -120,7 +120,7 @@ void BLO_memfile_write_init(MemFileWriteData *mem_data,
           *entry = mem_chunk;
         }
         else {
-          BLI_assert(0);
+          BLI_assert_unreachable();
         }
       }
     }

source/blender/blenloader/intern/writefile.c

@@ -1409,7 +1409,7 @@ bool BLO_write_file(Main *mainvar,
           BKE_bpath_absolute_convert(mainvar, dir_src, NULL);
           break;
         case BLO_WRITE_PATH_REMAP_NONE:
-          BLI_assert(0); /* Unreachable. */
+          BLI_assert_unreachable(); /* Unreachable. */
           break;
       }
     }