cmd/compile: remove nil check in accessing PAUTOHEAP variable

CL 23393 introduces PAUTOHEAP, and access of PAUTOHEAP variable is
rewritten to indirection of a PAUTO variable. Mark this variable
non-nil, so this indirection does not introduce extra nil checks.

Change-Id: I31853eed5e60238b6c5bc0546e2e9ab340dcddd9
Reviewed-on: https://go-review.googlesource.com/26831
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -506,7 +506,9 @@ func walkexpr(n *Node, init *Nodes) *Node {
 	if n.Op == ONAME && n.Class == PAUTOHEAP {
 		nn := Nod(OIND, n.Name.Heapaddr, nil)
 		nn = typecheck(nn, Erv)
-		return walkexpr(nn, init)
+		nn = walkexpr(nn, init)
+		nn.Left.NonNil = true
+		return nn
 	}
 
 opswitch: