compiler: ensure iterators are inlined in -opt=z

[eliasnaur]

$ cat iter_test.go
package inlineiter

import (
	"iter"
	"testing"
)

var slice = make([]int, 1000)

func TestIterAllocs(t *testing.T) {
	tests := []struct {
		name string
		body func() int
	}{
		{
			"for-range-func",
			func() int {
				sum := 0
				for v := range Iterator([]int{0, 1, 2}) {
					sum += v
				}
				return sum
			},
		},
		{
			"callback-func",
			func() int {
				sum := 0
				Iterator([]int{0, 1, 2})(func(v int) bool {
					sum += v
					return true
				})
				return sum
			},
		},
	}
	for _, test := range tests {
		t.Run(test.name, func(t *testing.T) {
			allocs := testing.Benchmark(func(b *testing.B) {
				for range b.N {
					sum := test.body()
					if sum != 3 {
						t.Errorf("sum %d, want 3", sum)
					}
				}
			}).AllocsPerOp()
			if allocs > 0 {
				t.Errorf("Layout allocates %d, expected %d", allocs, 0)
			}
		})
	}
}

func Iterator(ints []int) iter.Seq[int] {
	return func(yield func(int) bool) {
		for _, v := range ints {
			if !yield(v) {
				break
			}
		}
	}
}
$ go test
PASS
ok  	seedhammer.com/inlineiter	4.616s
$ tinygo test
--- FAIL: TestIterAllocs (3.08s)
    --- FAIL: TestIterAllocs/for-range-func (1.58s)
        Layout allocates 5, expected 0
    --- FAIL: TestIterAllocs/callback-func (1.50s)
        Layout allocates 3, expected 0
FAIL
FAIL	seedhammer.com/inlineiter	3.297s
$ tinygo test -opt 2
ok  	seedhammer.com/inlineiter	3.211s

The idiom for iterator constructors is to return a closure that captures the iterator arguments, if any. It's crucial for the performance of range-over-func that the constructor doesn't incur allocations. Therefore, it seems to me functions that (nearly) immediately return a closure should always be inlined, regardless of optimization level. Or, at the very least, any function that returns a function that can be ranged over.