compiler: make heap expression's write barrier conditional

compiler: make heap expression's write barrier conditional

    Heap_expression::do_get_backend emits an unconditional write
    barrier if the type has pointers and it is not a stack allocation.
    This CL changes it to use a write barrier for the assignment only
    when write barriers are enabled. While here, also change it to
    call gcWriteBarrier instead of typedmemmove for pointer-shaped
    types.

    For this to work, Function::build needs to be adjusted so that
    Heap_expression::do_get_backend is called when there is a parent
    block.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/181540

From-SVN: r272132