cmd/compile: inline checknil

Now that checknil has only a single caller, inline it.

Passes toolstash-check.

Change-Id: I5b13596bef84dd9a3e7f4bff8560903f1e54acfb
Reviewed-on: https://go-review.googlesource.com/c/148829
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/closure.go

@@ -523,8 +523,14 @@ func walkpartialcall(n *Node, init *Nodes) *Node {
 		// Trigger panic for method on nil interface now.
 		// Otherwise it happens in the wrapper and is confusing.
 		n.Left = cheapexpr(n.Left, init)
+		n.Left = walkexpr(n.Left, nil)
 
-		checknil(n.Left, init)
+		tab := nod(OITAB, n.Left, nil)
+		tab = typecheck(tab, ctxExpr)
+
+		c := nod(OCHECKNIL, tab, nil)
+		c.SetTypecheck(1)
+		init.Append(c)
 	}
 
 	typ := partialCallType(n)

src/cmd/compile/internal/gc/subr.go

@@ -1829,18 +1829,6 @@ func isbadimport(path string, allowSpace bool) bool {
 	return false
 }
 
-func checknil(x *Node, init *Nodes) {
-	x = walkexpr(x, nil) // caller has not done this yet
-	if x.Type.IsInterface() {
-		x = nod(OITAB, x, nil)
-		x = typecheck(x, ctxExpr)
-	}
-
-	n := nod(OCHECKNIL, x, nil)
-	n.SetTypecheck(1)
-	init.Append(n)
-}
-
 // Can this type be stored directly in an interface word?
 // Yes, if the representation is a single pointer.
 func isdirectiface(t *types.Type) bool {