[llvm] [RISCV] Move slide and gather costing to subtarget [NFC] (PR #65396)

[Philip Reames via llvm-commits]

preames wrote:

Landed as: 

```
commit 463c9f44dcd8a7a6e056cf3fd0a92ca42c8c6155 (HEAD -> main, upstream/main, upstream/HEAD)
Author: Philip Reames <preames at rivosinc.com>
Date:   Thu Sep 7 16:08:47 2023 -0700

    [RISCV] Move slide and gather costing to TLI [NFC] (PR #65396)
    
    As mentioned in TODOs from D159332.  This PR doesn't actually
    common up that copy of the code because doing so is not NFC - due to
    DLEN.  Fixing that will be a future PR.
```


https://github.com/llvm/llvm-project/pull/65396