Spread elements on spans should use "for-loop" lowering, not GetEnumerator

[RikkiGibson]

Repro: compile and inspect IL on the following: SharpLab

using System;
using System.Collections.Generic;

static HashSet<T> FromSpanSpread<T>(ReadOnlySpan<T> inputs)
{
    return [ .. inputs];
}

static HashSet<T> FromSpanManual<T>(ReadOnlySpan<T> inputs)
{
    HashSet<T> set = new(inputs.Length);
    foreach (T e in inputs)
    {
        set.Add(e);
    }
    
    return set;
}

Expected: both use "for-loop" lowering
Actual: "foreach" is lowered as "for", while the spread element uses GetEnumerator/MoveNext.

Looks like a roslyn bug. I see some bits in foreach lowering which are handling the span case and "rewriting as for", which are missed in the spread element lowering.

roslyn/src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_ForEachStatement.cs

Line 60 in 8e62064

else if (node.AwaitOpt is null && CanRewriteForEachAsFor(node.Syntax, nodeExpressionType, out var indexerGet, out var lengthGetter))

roslyn/src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_CollectionExpression.cs

Line 1402 in 8e62064

statement = RewriteForEachEnumerator(

Originally posted by @RikkiGibson in dotnet/runtime#111093 (comment)