cmd/gc: fix escape analysis ordering

cmd/gc: fix escape analysis ordering

Functions without bodies were excluded from the ordering logic,
because when I wrote the ordering logic there was no reason to
analyze them.

But then we added //go:noescape tags that need analysis, and we
didn't update the ordering logic.

So in the absence of good ordering, //go:noescape only worked
if it appeared before the use in the source code.

Fixes #5773.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/10570043