[llvm] [AArch64] Refactor and move common code to `AArch64PrologueEpilogue` (NFCI) (PR #158920)

[Benjamin Maxwell via llvm-commits]

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@@ -94,24 +137,16 @@ class AArch64PrologueEmitter {
   // emitFramePointerSetup.
   bool EmitCFI = false;
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MacDue wrote:

I've moved these common flags, but set them in the derived constructors (added some comments explaining where they're set too). 

https://github.com/llvm/llvm-project/pull/158920