compiler: do simple deadcode elimination

compiler: do simple deadcode elimination

    Normally the backend will do deadcode elimination and this is
    sufficient. However, the escape analysis operates on the AST that
    may have deadcode, and may cause things to escape that otherwise
    do not.

    This CL adds a simple deadcode elimination, run before the escape
    analysis.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/181080

From-SVN: r272043