[llvm] [AArch64] Ensure the LR is preserved if we must call __arm_get_current_vg (PR #145760)

[Benjamin Maxwell via llvm-commits]

================
@@ -3955,6 +3955,10 @@ void AArch64FrameLowering::determineCalleeSaves(MachineFunction &MF,
   // Save number of saved regs, so we can easily update CSStackSize later.
   unsigned NumSavedRegs = SavedRegs.count();
 
+  // If we must call __arm_get_current_vg in the prologue preserve the LR.
+  if (requiresSaveVG(MF) && !Subtarget.hasSVE())
+    SavedRegs.set(AArch64::LR);
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MacDue wrote:

Yes, as later it's used to update the `CSStackSize` to include the additional saves:

```
  // Adding the size of additional 64bit GPR saves.
  CSStackSize += 8 * (SavedRegs.count() - NumSavedRegs);
```

https://github.com/llvm/llvm-project/pull/145760