JIT: Keep FieldSeq in AssertionDesc

[EgorBo]

Closes #64876

This PR slightly changes memory layout in AssertionDsc (diff) while keeping total size the same (48 bytes) in order to save a reference to FieldSeq*.

Example:

static byte Foo()
{
    ReadOnlySpan<byte> RVA = new byte[] { 1, 2, 3, 4, 5 };
    return RVA[3];
}

JITDump diff: https://www.diffchecker.com/HVVahKbd e.g.:

This also is required to be able to fold constant index + RVA in future.

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Issue Details

---

Closes #64876

This PR slightly changes memory layout in AssertionDsc (diff) while keeping total size the same (48 bytes) in order to save a reference to FieldSeq*.

Example:

static byte Foo()
{
    ReadOnlySpan<byte> RVA = new byte[] { 1, 2, 3, 4, 5 };
    return RVA[3];
}

JITDump diff: https://www.diffchecker.com/HVVahKbd e.g.:

This also is required to be able to fold constant index + RVA in future.

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

[EgorBo]

@SingleAccretion @jakobbotsch please, take a look
I need this PR to land another one to fold RVA + constant index

[SingleAccretion]

LGTM modulo minor comments.