For calls that return multiple results we used to create a temporary
    of struct type to hold the results, and also create a separate
    temporary for each result.  Then the call expression would copy each
    result out of the struct to the temporary, and Call_result_expression
    would refer to the desired temporary.
    
    Simplify this to just use a single temporary of struct type, and
    change Call_result_expression to fetch a field of the struct.
    
    This may reduce some incorrect tree sharing in the backend code.
    
    Reviewed-on: https://go-review.googlesource.com/51770

From-SVN: r250682