cmd/compile: map literals should pass static size to constructor

[randall77]

func f(x, y, z int) map[int]int {
    return map[int]int{x: y, y: z, z: x}
}

This code calls runtime.makemap with 0 as the size hint. It should pass 3 as the size hint.

See #15880 for a similar issue about static maps.