ssa: support runtime.map

cl/_testrt/makemap/in.go

@@ -0,0 +1,12 @@
+package main
+
+func main() {
+	m := make(map[int]string)
+	m[1] = "hello"
+	m[2] = "world"
+	println(m[1], m[2])
+	v, ok := m[2]
+	println(v, ok)
+	v2, ok := m[10]
+	println(v2, ok)
+}