staticcheck: consider de facto constant expressions in SA9006 and other checks

[ainar-g]

(Inspired by a comment from @go101.)

Consider:

package main

const s = "Go101.org"
// len(s) == 9
// 1 << 9 == 512
// 512 / 128 == 4

var a byte = 1 << len(s) / 128
var b byte = 1 << len(s[:]) / 128

func main() {
	println(a, b)
}

$ go run ./tmp.go
4 0
$ go vet ./tmp.go
$
$ staticcheck --checks=all ./tmp.go
$

staticcheck does flag something like:

var c byte = 1
c <<= len(s)

But len(s[:]) is not constant, even though it can never change. Perhaps staticcheck could be taught to reason about these non-constant-but-immutable expressions.