compiler: use a single temporary for calls with multiple results

compiler: use a single temporary for calls with multiple results

    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