RBO: "X < Y" means "X + 1 <= Y"

[EgorBo]

Closes #112953

public static ReadOnlySpan<char> Test(ReadOnlySpan<char> span, int offset)
{
    if ((uint)offset < (uint)span.Length)
        return span.Slice(offset + 1);
    return span;
}

Codegen diff:

; Method Program:Test(System.ReadOnlySpan`1[ushort],int):System.ReadOnlySpan`1[ushort] (FullOpts)
-      sub      rsp, 40
       mov      rax, bword ptr [rdx]
       mov      edx, dword ptr [rdx+0x08]
       cmp      r8d, edx
       jb       SHORT G_M39302_IG04
       mov      bword ptr [rcx], rax
       mov      dword ptr [rcx+0x08], edx
       jmp      SHORT G_M39302_IG05
G_M39302_IG04:
       inc      r8d
-      cmp      r8d, edx
-      ja       SHORT G_M39302_IG07
       mov      r10d, r8d
       lea      rax, bword ptr [rax+2*r10]
       sub      edx, r8d
       mov      bword ptr [rcx], rax
       mov      dword ptr [rcx+0x08], edx
G_M39302_IG05:
       mov      rax, rcx
-      add      rsp, 40
       ret      
-G_M39302_IG07:
-      call     [System.ThrowHelper:ThrowArgumentOutOfRangeException()]
-      int3     
-; Total bytes of code: 62
+; Total bytes of code: 42