compiler: add break label in 1,2-case select statement lowering

compiler: add break label in 1,2-case select statement lowering

    CL 184998 added optimizations for one- and two-case select
    statements. But it didn't handle break statement in the select
    case correctly. Specifically, it didn't add the label definition,
    so it could result in a dangling goto. This CL fixes this, by
    adding the label definition.

    A test case is CL 185520.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/185519

From-SVN: r273359