Go patch committed: avoid orphaning Bexpressions when processing conversions

[Ian Lance Taylor]

The Go frontend method Type_conversion_expression::do_get_backend was
(in some circumstances) creating a Bexpression for the source
expression of the conversion and then throwing it away before using
it.  This patch by Than McIntosh fixes up this method to insure that
the call to get_backend() on the source expression is only made when
the result will be used.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline.

Ian

Index: gcc/go/gofrontend/MERGE
===================================================================
--- gcc/go/gofrontend/MERGE     (revision 249125)
+++ gcc/go/gofrontend/MERGE     (working copy)
@@ -1,4 +1,4 @@
-d4875b19266d5f726e0e32843b903075f5c50b4c
+61222d34c1b33a369bd86008a0541455dd17727e
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
Index: gcc/go/gofrontend/expressions.cc
===================================================================
--- gcc/go/gofrontend/expressions.cc    (revision 249125)
+++ gcc/go/gofrontend/expressions.cc    (working copy)
@@ -3410,11 +3410,13 @@ Type_conversion_expression::do_get_backe
 
   Gogo* gogo = context->gogo();
   Btype* btype = type->get_backend(gogo);
-  Bexpression* bexpr = this->expr_->get_backend(context);
   Location loc = this->location();
 
   if (Type::are_identical(type, expr_type, false, NULL))
-    return gogo->backend()->convert_expression(btype, bexpr, loc);
+    {
+      Bexpression* bexpr = this->expr_->get_backend(context);
+      return gogo->backend()->convert_expression(btype, bexpr, loc);
+    }
   else if (type->interface_type() != NULL
           || expr_type->interface_type() != NULL)
     {
@@ -3483,6 +3485,7 @@ Type_conversion_expression::do_get_backe
   else if (type->is_numeric_type())
     {
       go_assert(Type::are_convertible(type, expr_type, NULL));
+      Bexpression* bexpr = this->expr_->get_backend(context);
       return gogo->backend()->convert_expression(btype, bexpr, loc);
     }
   else if ((type->is_unsafe_pointer_type()
@@ -3493,7 +3496,10 @@ Type_conversion_expression::do_get_backe
            || (this->may_convert_function_types_
                && type->function_type() != NULL
                && expr_type->function_type() != NULL))
-    return gogo->backend()->convert_expression(btype, bexpr, loc);
+    {
+      Bexpression* bexpr = this->expr_->get_backend(context);
+      return gogo->backend()->convert_expression(btype, bexpr, loc);
+    }
   else
     {
       Expression* conversion =