Don't load slice values in a for range unless they're explicitly used in the for statement

[upamanyus]

This has a data race:

package main

func main() {
	sl := make([]string, 1)
	go func () {
		var x string
		_ = x
		for _, x = range sl {
		}
	}()
	sl[0] = ""
}

but changing the for loop to for _, _ = range sl { fixes it. Currently, Goose models both as a data race, because the same slice.for_range iterator is used, which requires loading the value of each element. This is a conservative semantics, but makes some programs difficult/impossible to prove (e.g. this is more annoying than necessary https://github.com/goose-lang/goose/blob/c12b8af04edef8ccdee729bbd0149f5161531987/testdata/examples/channel/workq/w.go#L75-L77)

(Copied from goose-lang/goose#188.)