stp/ldp optimization doesn't kick in for negative offsets

[EgorBo]

void Test(ref IntPtr ip)
{
    Unsafe.Add(ref ip, -1) = 0;
    Unsafe.Add(ref ip, -2) = 0;
    Unsafe.Add(ref ip, -3) = 0;
    Unsafe.Add(ref ip, -4) = 0;
}

Emits:

            str     xzr, [x1, #-0x08]
            str     xzr, [x1, #-0x10]
            str     xzr, [x1, #-0x18]
            str     xzr, [x1, #-0x20]

Expected:

            stp     xzr, xzr, [x1, #-0x08]
            stp     xzr, xzr, [x1, #-0x18]

Negative offsets are used e.g. in SpanHelpers.ClearWithReferences.

cc @kunalspathak @dotnet/jit-contrib

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Issue Details

---

void Test(ref IntPtr ip)
{
    Unsafe.Add(ref ip, -1) = 0;
    Unsafe.Add(ref ip, -2) = 0;
    Unsafe.Add(ref ip, -3) = 0;
    Unsafe.Add(ref ip, -4) = 0;
}

Emits:

            str     xzr, [x1, #-0x08]
            str     xzr, [x1, #-0x10]
            str     xzr, [x1, #-0x18]
            str     xzr, [x1, #-0x20]

Expected:

            stp     xzr, xzr, [x1, #-0x08]
            stp     xzr, xzr, [x1, #-0x18]

Negative offsets are used e.g. in SpanHelpers.ClearWithReferences.

cc @kunalspathak @dotnet/jit-contrib

Author:	EgorBo
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[kunalspathak]

This pattern was disabled in #90700. I will check with @TIHan to get the context of why we decided to disable it.