compiler: Don't crash on invalid arithmetic ops.

    
    The gofrontend would crash after hitting an unreachable state while
    trying to determine the type of an arithmetic expression involving
    non-numeric values.  Instead of crashing, it should fail gracefully
    if the relevant error is already reported.
    
    Fixes golang/go#11537.
    
    Reviewed-on: https://go-review.googlesource.com/13793

From-SVN: r227227

gcc/go/gofrontend/MERGE

-d5e6af4e6dd456075a1ec1c03d0dc41cbea5eb36
+cd5362c7bb0b207f484a8dfb8db229fd2bffef09
 
 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

@@ -15150,7 +15150,11 @@ Numeric_constant::set_type(Type* type, bool issue_error, Location loc)
   else if (type->complex_type() != NULL)
     ret = this->check_complex_type(type->complex_type(), issue_error, loc);
   else
-    go_unreachable();
+    {
+      ret = false;
+      if (issue_error)
+        go_assert(saw_errors());
+    }
   if (ret)
     this->type_ = type;
   return ret;