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

src/cmd/gc/esc.c

@@ -144,7 +144,7 @@ visitcode(Node *n, uint32 min)
 		fn = n->left;
 		if(n->op == OCALLMETH)
 			fn = n->left->right->sym->def;
-		if(fn && fn->op == ONAME && fn->class == PFUNC && fn->defn && fn->defn->nbody)
+		if(fn && fn->op == ONAME && fn->class == PFUNC && fn->defn)
 			if((m = visit(fn->defn)) < min)
 				min = m;
 	}

test/escape2.go

@@ -1337,3 +1337,22 @@ func foo143() {
 		}()
 	}
 }
+
+// issue 5773
+// Check that annotations take effect regardless of whether they
+// are before or after the use in the source code.
+
+//go:noescape
+
+func foo144a(*int)
+
+func foo144() {
+	var x int
+	foo144a(&x) // ERROR "&x does not escape"
+	var y int
+	foo144b(&y) // ERROR "&y does not escape"
+}
+
+//go:noescape
+
+func foo144b(*int)