compiler, runtime: open code select

compiler, runtime: open code select

    This is the gofrontend version of https://golang.org/cl/37933,
    https://golang.org/cl/37934, and https://golang.org/cl/37935.
    Open code the initialization of select cases.

    This is a step toward updating libgo to the 1.11 release.

    Reviewed-on: https://go-review.googlesource.com/135000

From-SVN: r264290