runtime: simplify code
runtime: simplify code

LGTM=khr
R=golang-codereviews, dave, khr
CC=golang-codereviews, rsc
https://codereview.appspot.com/116950043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go