Elide ldelema type check when array type is known exactly

[hamarb123]

Description

E.g.,

static ref object GetRef1(object obj)
{
    object[] arr = [obj];
    return ref arr[0];
}

static ref object GetRef2(object obj)
{
    object[] arr = [obj];
    return ref MemoryMarshal.GetArrayDataReference(arr);
}

These should be able to end up with the same codegen, but the first still generates the ldelema helper call, despite knowing that the array is exactly an object[] (not a string[] or similar).

Similarly, it could apply to static readonly fields once the static constructor has run.

Godbolt.org: https://godbolt.org/z/YGEh3zPMv

Codegen:

Program:GetRef1(System.Object):byref (FullOpts):
       push     r15
       push     rbx
       push     rax
       mov      rbx, rdi
       lea      rdi, [(reloc 0x432ec0)]      ; System.Object[]
       mov      esi, 1
       call     CORINFO_HELP_NEWARR_1_PTR
       mov      r15, rax
       lea      rdi, bword ptr [r15+0x10]
       mov      rsi, rbx
       call     CORINFO_HELP_ASSIGN_REF
       mov      rdi, r15
       xor      esi, esi
       lea      rdx, [(reloc 0x432fe8)]      ; System.Object
       call     CORINFO_HELP_LDELEMA_REF
       nop      
       add      rsp, 8
       pop      rbx
       pop      r15
       ret      

Program:GetRef2(System.Object):byref (FullOpts):
       push     r15
       push     rbx
       push     rax
       mov      rbx, rdi
       lea      rdi, [(reloc 0x432ec0)]      ; System.Object[]
       mov      esi, 1
       call     CORINFO_HELP_NEWARR_1_PTR
       mov      r15, rax
       lea      rdi, bword ptr [r15+0x10]
       mov      rsi, rbx
       call     CORINFO_HELP_ASSIGN_REF
       lea      rax, bword ptr [r15+0x10]
       add      rsp, 8
       pop      rbx
       pop      r15
       ret      

Regression?

No.

Analysis

Seems like #85256 handled it for sealed types in any scenario, but we can also handle it for when we know the type of the array object exactly.

May also be applicable for stelem.ref.

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