compiler: avoid call duplication during order eval.

    
    During the compiler's order_evaluations() pass, duplicate
    call statements were being created and inserted into block
    statement lists in situations where there were calls returning
    multiple arguments. These duplicates were benign in that they
    were ignored during backend code generation, however they
    did cause duplicate calls in AST dumps. Tweak the order
    evaluation code to handle this case better and avoid inserting
    these duplicates.
    
    Fixes golang/go#17237
    
    Reviewed-on: https://go-review.googlesource.com/29857

From-SVN: r240559

gcc/go/gofrontend/MERGE

-1d8d834b5eb9f683cc06529145b353bb5b08e7ea
+8aca265d317059ae6d9721a4a231895d80d0a82c
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.cc

@@ -3616,11 +3616,21 @@ Order_eval::statement(Block* block, size_t* pindex, Statement* s)
 	  // be handled specially.  We can't create a temporary
 	  // because there is no type to give it.  Any actual uses of
 	  // the values will be done via Call_result_expressions.
-	  s = Statement::make_statement(*pexpr, true);
-	}
+          //
+          // Since a given call expression can be shared by multiple
+          // Call_result_expressions, avoid hoisting the call the
+          // second time we see it here.
+          if (this->remember_expression(*pexpr))
+            s = NULL;
+          else
+            s = Statement::make_statement(*pexpr, true);
+        }
 
-      block->insert_statement_before(*pindex, s);
-      ++*pindex;
+      if (s != NULL)
+        {
+          block->insert_statement_before(*pindex, s);
+          ++*pindex;
+        }
     }
 
   if (init != orig_init)
@@ -7949,13 +7959,14 @@ Traverse::remember_type(const Type* type)
 }
 
 // Record that we are looking at an expression, and return true if we
-// have already seen it.
+// have already seen it. NB: this routine used to assert if the traverse
+// mask did not include expressions/types -- this is no longer the case,
+// since it can be useful to remember specific expressions during
+// walks that only cover statements.
 
 bool
 Traverse::remember_expression(const Expression* expression)
 {
-  go_assert((this->traverse_mask() & traverse_types) != 0
-	     || (this->traverse_mask() & traverse_expressions) != 0);
   if (this->expressions_seen_ == NULL)
     this->expressions_seen_ = new Expressions_seen();
   std::pair<Expressions_seen::iterator, bool> ins =