[llvm] [AArch64][SME2] Preserve ZT0 state around function calls (PR #76968)

[Sander de Smalen via llvm-commits]

================
@@ -36,7 +36,10 @@ class SMEAttrs {
     ZA_New = 1 << 4,        // aarch64_pstate_sm_new
     ZA_Preserved = 1 << 5,  // aarch64_pstate_sm_preserved
     ZA_NoLazySave = 1 << 6, // Used for SME ABI routines to avoid lazy saves
-    All = ZA_Preserved - 1
+    ZT_New = 1 << 7,        // aarch64_sme_pstate_zt0_new
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sdesmalen-arm wrote:

```suggestion
    ZT0_New = 1 << 7,        // aarch64_sme_zt0_new
```
(missing the `0` in `ZT0` and remove the `pstate_` from the comment, see my suggestion above for rationale)

https://github.com/llvm/llvm-project/pull/76968