[AArch64] Refactor checks in sign-return-address.ll test

Using implicit CHECK prefix in one FileCheck invocation and explicit
CHECK-V83A in the other one seems to misguide to use CHECK: lines as
a common matching prefix at various places. Also note that

; CHECK, CHECK-V83A: ...

line only matches the "CHECK-V83A" prefix.

This commit explicitly splits the checks into common ones (CHECK) and
invocation-specific ones (COMPAT and V83A) and updates the assertions
with the update_llc_test_checks.py script.

Reviewed By: efriedma, MaskRay

Differential Revision: https://reviews.llvm.org/D156327