compiler: Report errors after evaluating invalid constant.

    
    The fix for golang/go#11541 made the assertion that getting an invalid
    Numeric_constant after evaluating an invalid constant must result in
    an error in a binary expression.  However, a Numeric_constant can be
    invalid if it is unset, which occurs when either operand cannot be
    converted to the right type.  These errors are issued after the
    Numeric_constant is lowered.
    
    Fixes golang/go#12615.
    
    Reviewed-on: https://go-review.googlesource.com/14646

From-SVN: r229098

gcc/go/gofrontend/MERGE

-302d8abbc499e28088d758ae8b2c024d8e50b9b3
+11e249a59e8c627fe9c2938c38e39cb1efefb1fb
 
 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

@@ -4909,14 +4909,7 @@ Binary_expression::do_lower(Gogo* gogo, Named_object*,
 	    Numeric_constant nc;
 	    if (!Binary_expression::eval_constant(op, &left_nc, &right_nc,
 						  location, &nc))
-              {
-                if (nc.is_invalid())
-                  {
-                    go_assert(saw_errors());
-                    return Expression::make_error(location);
-                  }
                 return this;
-              }
 	    return nc.expression(location);
 	  }
       }