JIT: Expand recognition of struct field offset pattern

[jakobbotsch]

The JIT has an optimization to recognize recomputations of field offsets, but this optimization failed to trigger in certain cases when the struct was promoted with old promotion.

public unsafe nint Test()
{
    MyStruct dummy;
    return (nint)((byte*)&dummy.Field - (byte*)&dummy);
}

private struct MyStruct
{
    public int A;
    public int Field;
}

Before:

G_M27992_IG01:  ;; offset=0x0000
       push     rax
       xor      eax, eax
       mov      qword ptr [rsp], rax
						;; size=7 bbWeight=1 PerfScore 2.25

G_M27992_IG02:  ;; offset=0x0007
       lea      rax, [rsp+0x04]
       lea      rcx, [rsp]
       sub      rax, rcx
						;; size=12 bbWeight=1 PerfScore 1.25

G_M27992_IG03:  ;; offset=0x0013
       add      rsp, 8
       ret
						;; size=5 bbWeight=1 PerfScore 1.25

After:

G_M27992_IG02:  ;; offset=0x0000
       mov      eax, 4
						;; size=5 bbWeight=1 PerfScore 0.25

G_M27992_IG03:  ;; offset=0x0005
       ret
						;; size=1 bbWeight=1 PerfScore 1.00

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[jakobbotsch]

PTAL @dotnet/jit-contrib

Minor improvements

[Copilot]

Pull request overview

This PR enhances the JIT compiler's optimization to recognize and fold struct field offset computations when structs are promoted using old promotion. Previously, the optimization only worked when computing offsets between the same local variable, but failed when promoted struct fields were involved.

• Extends the SUB node optimization to resolve parent local variables and aggregate field offsets for promoted struct fields
• Enables compile-time constant folding of field offset patterns that previously required runtime computation
• Significantly improves code generation by eliminating unnecessary address arithmetic