test: use go:noinline consistently

Replace various implementations of inlining prevention with
"go:noinline"

Change-Id: Iac90895c3a62d6f4b7a6c72e11e165d15a0abfa4
Reviewed-on: https://go-review.googlesource.com/16510
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/test/fixedbugs/issue4518.go b/test/fixedbugs/issue4518.go
index e64b069..5c3a178 100644
--- a/test/fixedbugs/issue4518.go
+++ b/test/fixedbugs/issue4518.go
@@ -10,15 +10,13 @@
 
 package main
 
-func DontInline() {}
-
+//go:noinline
 func F(e interface{}) (int, int) {
-	DontInline()
 	return 3, 7
 }
 
+//go:noinline
 func G() (int, int) {
-	DontInline()
 	return 3, 7
 }