cmd/compile: dead code-elimination should work for sync.OnceValue

[dsnet]

This is special case of #14840, but identified as a potential source of special-cases if the general case is hard to solve.

Compiling the following program:

package main

import "sync"

var initializeVariable = sync.OnceValue(func() string {
    return "THIS_IS_DEAD_CODE"
})

func main() {}

should not link in logic for initializeVariable since the variable is dead.
This also applies to OnceFunc and OnceValues.

At present, OnceValue is unfortunately not a drop-in replacement for for sync.Once for initializing global variables.

[mknyszek]

CC @dr2chase who has been thinking about function purity analysis, which may be related.

[aclements]

A related optimization we should consider is detecting that the package variable is never modified and then applying inlining at sites that call the function bound to it. We do this for closures bound to local variables, but not at package scope. (This might be a different issue, or could be thought of together with this one.)