[llvm] [AArch64][SME] Implement the SME ABI (ZA state management) in Machine IR (PR #149062)

[Benjamin Maxwell via llvm-commits]

================
@@ -128,7 +129,7 @@ class AArch64Subtarget final : public AArch64GenSubtargetInfo {
                    unsigned MinSVEVectorSizeInBitsOverride = 0,
                    unsigned MaxSVEVectorSizeInBitsOverride = 0,
                    bool IsStreaming = false, bool IsStreamingCompatible = false,
-                   bool HasMinSize = false);
+                   bool HasMinSize = false, bool UseNewSMEABILowering = false);
----------------
MacDue wrote:

I can just add a static method  `AArch64TargetMachine::useNewSMEABILowering()` to return the flag, that should work. 

https://github.com/llvm/llvm-project/pull/149062