Recognize "Span.Length" during physical promotion and set IsNeverNegative flag

[EgorBo]

Closes #104573

[Copilot]

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

[EgorBo]

@jakobbotsch cc @dotnet/jit-contrib should we merge it as is then? Diffs (seems to be overall PerfScore improvement)

The end goal is to force spans being promoted for this snippet (win-x64):

static void Test(Span<int> span, int len)
{
    for (int i = 0; i < len; i++)
        span[i] = 0;
}

while currently (thanks to the old promotion) they're not.

[jakobbotsch]

I would obviously like to take this change right now, but I'm worried about regressions like

They happen because new promotion never considers it beneficial to promote structs that are only used "whole", while it turns out that can be beneficial because it can reduce the number of struct copies (instead of "memory -> stack, stack -> memory" the promotion turns things into "memory -> registers -> memory").
It's a known deficiency right now and also shows up very frequently when I look at the diffs from disabling old promotion entirely. The fix for that is going to be something that still chooses to promote some of these only-whole-uses structs, but of course getting this right is probably also not going to be super simple...

Ideally we would allow physical promotion to take over when old promotion decides to undo promotion, but that also seems like a bunch of work...

[EgorBo]

@jakobbotsch Ok, I trust your instincts, how about merging the IsNeverNegative fix only? (changed) Diffs

[jakobbotsch]

That sounds good to me, can you fix the PR title?