exp/ssa: support multiple labels on same statement.

Actually it already worked since the spec only requires that
the one immediately preceding a for/switch/... be usable as
the target of a break or continue statement.

Added a test.
Also: allocate Function.lblocks on first use.

R=gri
CC=golang-dev
https://golang.org/cl/7365058

src/pkg/exp/ssa/builder.go

@@ -2215,7 +2215,6 @@ func (b *Builder) stmt(fn *Function, _s ast.Stmt) {
 	// target is always set; its _break and _continue are set only
 	// within the body of switch/typeswitch/select/for/range.
 	// It is effectively an additional default-nil parameter of stmt().
-	// TODO(adonovan): fix: handle multiple labels on the same stmt.
 	var label *lblock
 start:
 	switch s := _s.(type) {

src/pkg/exp/ssa/func.go

@@ -152,6 +152,9 @@ func (f *Function) labelledBlock(label *ast.Ident) *lblock {
 	lb := f.lblocks[label.Obj]
 	if lb == nil {
 		lb = &lblock{_goto: f.newBasicBlock(label.Name)}
+		if f.lblocks == nil {
+			f.lblocks = make(map[*ast.Object]*lblock)
+		}
 		f.lblocks[label.Obj] = lb
 	}
 	return lb
@@ -200,7 +203,6 @@ func (f *Function) start(idents map[*ast.Ident]types.Object) {
 	if f.syntax == nil {
 		return // synthetic function; no syntax tree
 	}
-	f.lblocks = make(map[*ast.Object]*lblock)
 
 	// Receiver (at most one inner iteration).
 	if f.syntax.recvField != nil {

src/pkg/exp/ssa/interp/testdata/coverage.go

@@ -363,3 +363,30 @@ func init() {
 		panic("I.f not called twice")
 	}
 }
+
+// Multiple labels on same statement.
+func multipleLabels() {
+	var trace []int
+	i := 0
+one:
+two:
+	for ; i < 3; i++ {
+		trace = append(trace, i)
+		switch i {
+		case 0:
+			continue two
+		case 1:
+			i++
+			goto one
+		case 2:
+			break two
+		}
+	}
+	if x := fmt.Sprint(trace); x != "[0 1 2]" {
+		panic(x)
+	}
+}
+
+func init() {
+	multipleLabels()
+}