[llvm] [IR] Encode no-op addrspacecast validity in DataLayout (PR #210888)

[Matt Arsenault via llvm-commits]

arsenm wrote:

I don't really like this. The only reason this is a problem is the artificial restriction that you're not allowed to bitcast between pointers with different address spaces, which [we should allow](https://discourse.llvm.org/t/rfc-llvm-ir-should-allow-bitcast-between-address-spaces-with-the-same-size/5759/22?page=2)

https://github.com/llvm/llvm-project/pull/210888