Use number of slots intead of size in BuildBlockStore

[EgorBo]

Proper fix for #99611

The #99611 crash was caused by a semi-valid struct defined in Microsoft.macOS.dll:

    [StructLayout(LayoutKind.Explicit, Size = 28)]
    private struct sockaddr_in
    {
      [FieldOffset(0)]
      private byte sin_len;
      [FieldOffset(1)]
      private byte sin_family;
      [FieldOffset(2)]
      private short sin_port;
      [FieldOffset(4)]
      private int sin_addr;
      [FieldOffset(4)]
      private uint sin6_flowinfo;
      [MarshalAs(UnmanagedType.ByValArray, SizeConst = 16)]
      [FieldOffset(8)]
      public byte[] sin6_addr8;
      [FieldOffset(24)]
      private uint sin6_scope_id;
    }

the size was explicitly set to 28 while in fact it's rounded to 32 anyway due to GC ref.
The bug basically is:

1. LSRA: size is 28? then you don't need SIMD regs to copy it
2. Codegen: struct has 4 slots, give me 2 SIMD regs

[EgorBo]

Although, it's possible that the problem is actually on NAOT size - what should getClassSize API return for this struct? cc @jkotas @MichalStrehovsky

28 (as requested via Size=28) or 32 (rounded to 8 because of gc field)?

[EgorBo]

Yeah, just checked - CoreCLR returns 32 here. NAOT - 28.

[jkotas]

Yeah, just checked - CoreCLR returns 32 here. NAOT - 28.

Yes, this looks like a bug in native AOT. Opened #100220

[EgorBo]

Closing since this change shouldn't be needed with proper fix on VM side