Merge two functions that create sideffect free compound load nodes.

runtime/vm/parser.h

Index: runtime/vm/parser.h
===================================================================
--- runtime/vm/parser.h	(revision 7191)
+++ runtime/vm/parser.h	(working copy)
@@ -442,7 +442,7 @@
                               Token::Kind assignment_op,
                               AstNode* lhs,
                               AstNode* rhs);
-  AstNode* AsSideEffectFreeNode(AstNode* node);
+  AstNode* PrepareCompoundAssignmentNodes(AstNode** expr);
   LocalVariable* CreateTempConstVariable(intptr_t token_index,
                                          intptr_t token_id,
                                          const char* s);