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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

Note: I also played around with the idea of moving the VG saves out of frame lowering in #145748 (which I think generally simplifies the implementation), but it's probably too much churn to be worth considering. 

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