cmd/compile: preserve escape information for map literals

While some map literals were marked non-escaping that information
was lost when creating the corresponding OMAKE node which made map
literals always heap allocated.

Copying the escape information to the corresponding OMAKE node allows
stack allocation of hmap and a map bucket for non escaping map literals.

Fixes #21830

Change-Id: Ife0b020fffbc513f1ac009352f2ecb110d6889c9
Reviewed-on: https://go-review.googlesource.com/62790
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -931,6 +931,7 @@ func slicelit(ctxt initContext, n *Node, var_ *Node, init *Nodes) {
 func maplit(n *Node, m *Node, init *Nodes) {
 	// make the map var
 	a := nod(OMAKE, nil, nil)
+	a.Esc = n.Esc
 	a.List.Set2(typenod(n.Type), nodintconst(int64(n.List.Len())))
 	litas(m, a, init)
 

src/runtime/map_test.go

@@ -650,6 +650,13 @@ var testNonEscapingMapVariable int = 8
 
 func TestNonEscapingMap(t *testing.T) {
 	n := testing.AllocsPerRun(1000, func() {
+		m := map[int]int{}
+		m[0] = 0
+	})
+	if n != 0 {
+		t.Fatalf("mapliteral: want 0 allocs, got %v", n)
+	}
+	n = testing.AllocsPerRun(1000, func() {
 		m := make(map[int]int)
 		m[0] = 0
 	})