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

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@270862 138bc75d-0d04-0410-961f-82ee72b054a4