compiler: recognize and optimize map range clear

compiler: recognize and optimize map range clear

    Recognize

            for k := range m { delete(m, k) }

    for map m, and rewrite it to runtime.mapclear, as the gc compiler
    does.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/169397

From-SVN: r270780