Fix mem usage tracking of semir (#4684)

The call got misplaced during refactoring.

toolchain/driver/compile_subcommand.cpp

@@ -442,10 +442,6 @@ class CompilationUnit {
 
     sem_ir_.emplace(&*parse_tree_, check_ir_id, parse_tree_->packaging_decl(),
                     value_stores_, input_filename_);
-    if (mem_usage_) {
-      mem_usage_->Collect("sem_ir_", *sem_ir_);
-    }
-
     sem_ir_converter_.emplace(node_converters, &*sem_ir_);
     return {.consumer = consumer_,
             .value_stores = &value_stores_,
@@ -465,6 +461,10 @@ class CompilationUnit {
     // to wait for code generation.
     consumer_->Flush();
 
+    if (mem_usage_) {
+      mem_usage_->Collect("sem_ir_", *sem_ir_);
+    }
+
     if (options_.dump_raw_sem_ir && IncludeInDumps()) {
       CARBON_VLOG("*** Raw SemIR::File ***\n{0}\n", *sem_ir_);
       sem_ir_->Print(driver_env_->output_stream, options_.builtin_sem_ir);