Rename invoke-direct-empty to invoke-object-init

The invoke-direct-empty instruction was introduced to remove the
overhead of calling the empty Object constructor.  We now need it
to do some extra work on behalf of object construction, so it's
appropriate to change the instruction name to match the role it
fills rather than the more general role it was hoped to fill.

No functional changes.

Bug 3342343

Change-Id: I65dd6a2c00c99581c9a19b16fe193b70642c8fbb
diff --git a/vm/compiler/Dataflow.c b/vm/compiler/Dataflow.c
index d935fd5..ca72a68 100644
--- a/vm/compiler/Dataflow.c
+++ b/vm/compiler/Dataflow.c
@@ -750,7 +750,7 @@
     // EF OP_EXECUTE_INLINE_RANGE
     DF_FORMAT_3RC,
 
-    // F0 OP_INVOKE_DIRECT_EMPTY
+    // F0 OP_INVOKE_OBJECT_INIT
     DF_NOP,
 
     // F1 OP_RETURN_VOID_BARRIER