compiler: recognize and optimize array range clear

compiler: recognize and optimize array range clear

    Recognize

            for i := range a { a[i] = zero }

    for array or slice a, and rewrite it to call memclr, as the gc
    compiler does.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/169398

From-SVN: r270862