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

[Benjamin Maxwell via llvm-commits]

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@@ -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);
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MacDue wrote:

I did it this way because the flag needs to be defined in `AArch64TargetMachine.cpp` (to alter the pass pipeline), and this seemed like a fairly easy way to pass that information to ISel lowering. 

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