compiler: support comma-ok in map lookup

testdata/map.go

@@ -24,6 +24,8 @@ func main() {
 	readMap(testmap2, "ten")
 	delete(testmap2, "six")
 	readMap(testmap2, "seven")
+	lookup(testmap2, "eight")
+	lookup(testmap2, "nokey")
 }
 
 func readMap(m map[string]int, key string) {
@@ -33,3 +35,7 @@ func readMap(m map[string]int, key string) {
 		println(" ", k, "=", v)
 	}
 }
+func lookup(m map[string]int, key string) {
+	value, ok := m[key]
+	println("lookup with comma-ok:", key, value, ok)
+}