Surprising codegen for passing collection spread to span/readonlyspan taking method.

[CyrusNajmabadi]

Consider this code:

using System;
using System.Collections.Generic;

class C
{
    void Main(IEnumerable<string> values)
    {
        Goo([.. values, .. values]);
    }

    void Goo(ReadOnlySpan<string> spans)
    {
    }
}

Sharplab: https://sharplab.io/#v2:C4LglgNgNAJiDUAfAAgJgIwFgBQyAMABMugCwDcOOaBAwjgN44HNEkECyAhmAHYAUxAMwAeYngB8BAG6cIAVwCmAZwCUTFo2wttBAOIB7fXwDaAOlPTZipVALnL85QF0VFLSwC+ld82RsDRgBKCpwwAPI8EACeAMoADpw8ougSBEoJPKrqzJraXtgeQA

We generate the equivalent of:

	List<string> list = new List<string>();
	list.AddRange(values);
	list.AddRange(values);
	Goo(new ReadOnlySpan<string>(list.ToArray()));

The copy to an array seems unnecessary here, and more ideal codegen would seem to be:

	List<string> list = new List<string>();
	list.AddRange(values);
	list.AddRange(values);
	Goo(System.Runtime.InteropServices.CollectionsMarshal.AsSpan(list));

This would prevent a needless copy. Note that this should be entirely safe as:

1. The backing list is an impl detail hidden from the user (they cannot mutate it).
2. writing back into the list through the span would continue to be supported, but would also continue to be pointless.

This seems like a safe, and desirable change to make (as long as CollectionsMarshal.AsSpan is available) whenever we are targetting a span/read-only-span with a variable length number of elements.

[CyrusNajmabadi]

@RikkiGibson i was surprised by this. I thought we did this optimization already.

[RikkiGibson]

This looks like a dupe of #75863.