[llvm] Reland "[AArch64][SME] Split SMECallAttrs out of SMEAttrs" (PR #138671)

[Sander de Smalen via llvm-commits]

================
@@ -126,27 +123,83 @@ class SMEAttrs {
   bool isPreservesZT0() const {
     return decodeZT0State(Bitmask) == StateValue::Preserved;
   }
-  bool isUndefZT0() const { return Bitmask & ZT0_Undef; }
+  bool hasUndefZT0() const { return Bitmask & ZT0_Undef; }
   bool sharesZT0() const {
     StateValue State = decodeZT0State(Bitmask);
     return State == StateValue::In || State == StateValue::Out ||
            State == StateValue::InOut || State == StateValue::Preserved;
   }
   bool hasZT0State() const { return isNewZT0() || sharesZT0(); }
-  bool requiresPreservingZT0(const SMEAttrs &Callee) const {
-    return hasZT0State() && !Callee.isUndefZT0() && !Callee.sharesZT0() &&
-           !Callee.hasAgnosticZAInterface();
+
+  SMEAttrs operator|(SMEAttrs Other) const {
+    SMEAttrs Merged(*this);
+    Merged.set(Other.Bitmask, /*Enable=*/true);
----------------
sdesmalen-arm wrote:

nit:
```suggestion
    Merged.set(Other.Bitmask);
```

https://github.com/llvm/llvm-project/pull/138671