cmd/gc: don't walk static nodes generated by anylit.

During anylit run, nodes such as SLICEARR(statictmp, [:])
may be generated and are expected to be found unchanged by
gen_as_init.

In some walks (in particular walkselect), the statement
may be walked again and lowered to its usual form, leading to a
crash.

Fixes issue 8017.
Fixes issue 8024.
Fixes issue 8058.

LGTM=rsc
R=golang-codereviews, dvyukov, gobot, rsc
CC=golang-codereviews
https://codereview.appspot.com/112080043