compiler: avoid orphaning Bexpressions when processing conversions

compiler: avoid orphaning Bexpressions when processing conversions

    The method Type_conversion_expression::do_get_backend was (in some
    circumstances) creating a Bexpression for the source expression of the
    conversion and then throwing it away before using it. Fix up this
    method to insure that the call to get_backend() on the source
    expression is only made when the result will be used.

    Reviewed-on: https://go-review.googlesource.com/45350

From-SVN: r249131