[release/8.0] [JIT] ARM64 - Temporary fix for ldp/stp optimizations - with test fix

[github-actions]

Backport of #90700 to release/8.0

/cc @TIHan

Customer Impact

JIT produces bad code-gen which leads to invalid results.

Testing

Two tests were added to ensure we get the correct results.

Risk

Risk is very low as the likelihood of encountering the original bug was already very low. The change only impacts the original scenario.

Description

Resolves #85765

With the latest, the code-gen is quite different from what was reported in the issue, and therefore doesn't reproduce. But the issue still exists and is able to be reproduced by a different sample:

using System;
using System.Runtime.CompilerServices;

// Expected: 515
// Actual: 0
public unsafe class Program
{
    public static void Main()
    {
        byte* bytes = stackalloc byte[1024];
        bytes[0x1A] = 1;
        bytes[0x1B] = 2;
        int sum = Foo(bytes);
        Console.WriteLine(sum);
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    public static int Foo(byte* b)
    {
        return Unsafe.ReadUnaligned<int>(ref b[0x1A]) + Unsafe.ReadUnaligned<int>(ref b[0x1B]);
    }
}

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Issue Details

---

Backport of #90700 to release/8.0

/cc @TIHan

Customer Impact

Testing

Risk

IMPORTANT: If this backport is for a servicing release, please verify that:

• The PR target branch is release/X.0-staging, not release/X.0.

• If the change touches code that ships in a NuGet package, you have added the necessary package authoring and gotten it explicitly reviewed.

Author:	github-actions[bot]
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[jeffschwMSFT]

approved. please get a code review and we can merge

[TIHan]

@dotnet/jit-contrib this is ready - was already reviewed before

[JulieLeeMSFT]

CI errors are known issues.