&x.f, &x[0], x[i:j], &*x all must panic if x==nil.
The first three are already addressed by the semantics of
FieldAddr, IndexAddr, Slice; updated docs to reflect this.
The final case requires generation of an additional dynamic check.
See golang.org/s/go12nil for details.
Tested on $GOROOT/test/nilptr2.go (with patch from CL 13108043)
Also: remove a TODO where a no-op will do.
R=gri, crawshaw
CC=golang-dev, rsc
https://golang.org/cl/13064044
5cbf2abd36