[llvm] [AArch64][SME2] Add ZT0 attributes to SMEAttrs (PR #77607)

[Sander de Smalen via llvm-commits]

================
@@ -92,13 +97,22 @@ class SMEAttrs {
   }
 
   // Interfaces to query ZT0 State
-  bool hasNewZT0Body() const { return Bitmask & ZT0_New; }
-  bool isZT0In() const { return Bitmask & ZT0_In; }
-  bool isZT0Out() const { return Bitmask & ZT0_Out; }
-  bool isZT0InOut() const { return Bitmask & ZT0_InOut; }
-  bool preservesZT0() const { return Bitmask & ZT0_Preserved; }
+  StateValue getZT0State() const {
+    return static_cast<StateValue>((Bitmask & ZT0_Mask) >> ZT0_Shift);
+  }
+  void setZT0State(StateValue S) {
+    Bitmask |= (static_cast<unsigned>(S) << ZT0_Shift);
+  }
----------------
sdesmalen-arm wrote:

To enable the use of the `SMEAttrs` constructor using e.g. `SMEAttrs(<mask>)`, perhaps it makes more sense to change `setZT0State` to something that encodes the value given a bitmask, i.e.

```
  static unsigned decodeZT0State(unsigned Bitmask) {
    return static_cast<StateValue>((Bitmask & ZT0_Mask) >> ZT0_Shift);
  }
  static unsigned encodeZT0State(StateValue S) {
    return static_cast<unsigned>(S) << ZT0_Shift;
  }
```

That avoids having to do:
```
   SA ZT0_In = SA(SA::Normal);
   ZT0_In.setZT0State(SA::StateValue::In);
```

and instead allows:
```
SA ZT0_In = SA(SA::encodeZT0State(SA::StateValue::In));
```
without the extra level of indirection.

https://github.com/llvm/llvm-project/pull/77607