fixup! [AArch64][llvm] Unify AArch64 tests into a single file (4/4) (NFC)

This is a series of patches (4/4) to unify assembly/disassembly of
recent AArch64 tests into a single file. The aim is to improve
consistency, so that all instructions and system registers are
thoroughly tested, and future test cases will be in a unified format.

This patch:
 * removes .txt tests whose .s tests have functions
 * makes the .s tests have a roundabout run line to test both encoding and assembly

Co-authored-by: Virginia Cangelosi <virginia.cangelosi@arm.com>

Author: jthackray