[Experiment] Fold NativeAOT relocatable constants with small offsets

[EgorBo]

Just an AI experiment for relocatable constants + small offsets in NativeAOT.

using System.Runtime.CompilerServices;

Test();

class TestClass
{
    [MethodImpl(MethodImplOptions.NoInlining)]
    static ReadOnlySpan<char> Test() => "Hello".AsSpan();
}

-       lea      rax, gword ptr [(reloc ...)]      ; '"Hello"'
-       add      rax, 12
+       lea      rax, bword ptr [(reloc ...)]
        mov      bword ptr [rcx], rax
        mov      dword ptr [rcx+0x08], 5
        mov      rax, rcx
        ret

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[Copilot]

Pull request overview

This PR extends the x64 JIT NativeAOT pipeline to fold OBJ_HDL + small_const address computations into a single relocatable constant with an explicit “addend”, enabling codegen to avoid emitting a separate add reg, imm in common patterns (e.g., string literal span construction).

Changes:

• Teach Lowering::LowerAdd to fold GTF_ICON_OBJ_HDL + int32 offset into a new GTF_ICON_RELOC_ADDR constant (AMD64 + NativeAOT only), carrying the addend separately.
• Introduce GTF_ICON_RELOC_ADDR and add GenTreeIntCon accessors to store/retrieve the relocation addend.
• Extend the xarch emitter/codegen so lea reg, [reloc] can record a relocation with an extra addend (via a new emitIns_R_AI parameter and emitOutputAM handling).

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/coreclr/jit/lower.cpp	Adds the NativeAOT AMD64 folding path from OBJ_HDL + small offset into a relocatable constant with addend.
src/coreclr/jit/handlekinds.h	Adds the new handle kind GTF_ICON_RELOC_ADDR.
src/coreclr/jit/gentree.h	Documents and implements addend storage/access (GetRelocOffset/SetRelocOffset) on GenTreeIntCon.
src/coreclr/jit/gentree.cpp	Prevents address containment for GTF_ICON_RELOC_ADDR to avoid losing the addend.
src/coreclr/jit/emitxarch.h	Extends emitIns_R_AI signature with an optional relocation addend parameter.
src/coreclr/jit/emitxarch.cpp	Stores the addend in the instruction descriptor and applies it to RIP-rel relocation emission (emitOutputAM).
src/coreclr/jit/emitinl.h	Adds emitGetInsAmdRelocOffset helper to retrieve the stored addend.
src/coreclr/jit/emit.h	Declares emitGetInsAmdRelocOffset on the emitter.
src/coreclr/jit/codegenxarch.cpp	Emits lea with relocation addend for GTF_ICON_RELOC_ADDR (NativeAOT AMD64), with fallback to lea/mov + add.