[AArch64] Async unwind - Refactor generation of shadow call stack prologue/epilogue

This patch is in preparation for the async unwind CFI.

Move the emission of the shadow call stack prologue/epilogue
instructions to the `emitPrologue`/`emitEpilogue`. This greatly
simplifies especially epilogue generation and makes unnecessary some
quite fragile code, that tries to skip over those

Reviewed By: MaskRay, efriedma

Differential Revision: https://reviews.llvm.org/D112329