compiler: Use context to determine types of complex expressions.

When determining the type of a complex expression, it is important
    to recognize cases where a complex value can be represented as a
    real number.
    
    Fixes golang/go#11572.
    
    Reviewed-on: https://go-review.googlesource.com/12541

From-SVN: r226596

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-bdd98c601f2c8dbd0bf821548ba09c038f7645c4
+df080adb06f0e423820f3f6b9604b0c1093ff20a
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -2396,7 +2396,7 @@ class Complex_expression : public Expression
 
   void
   do_dump_expression(Ast_dump_context*) const;
-  
+
  private:
   // The complex value.
   mpc_t val_;
@@ -2423,8 +2423,7 @@ Complex_expression::do_determine_type(const Type_context* context)
 {
   if (this->type_ != NULL && !this->type_->is_abstract())
     ;
-  else if (context->type != NULL
-	   && context->type->complex_type() != NULL)
+  else if (context->type != NULL && context->type->is_numeric_type())
     this->type_ = context->type;
   else if (!context->may_be_abstract)
     this->type_ = Type::lookup_complex_type("complex128");