cmd/compile: slow bitmap generation for large array of zero-size elements

This (nonsense, but valid) program takes a very long time to compile because of stack bitmap generation:

    package f

    func F() {
        var x struct {
                x *int
                w [1e10][1e10][1e10][0]*int
                y *int
        }
        println(&x)
    }

The easy fix is to short-circuit generating stack bitmaps for zero-size types.