Afterprologue check was required when did not know
about return arguments of functions and/or they were not zeroed.
Now 100% precision is required for stacks due to stack copying,
so it must work w/o afterprologue one way or another.
I can limit this change for 1.3 to merely adding a TODO,
but this check is super confusing so I don't want this knowledge to get lost.

LGTM=rsc
R=golang-codereviews, gobot, rsc, khr
CC=golang-codereviews, khr, rsc
https://golang.org/cl/96580045