compiler: open code string slice expressions

compiler: open code string slice expressions

    Currently a string slice expression is implemented with a runtime
    call __go_string_slice. Change it to open code it, which is more
    efficient, and allows the backend to further optimize it.

    Also omit the write barrier for length-only update (i.e.
    s = s[:n]).

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/182540

From-SVN: r272549