[ELF][AArch64] Add test case for load/store exclusive instructions [NFC]

The AArch64 -fix-cortex-a53-843419 is missing a test case for the load and
store exclusive instructions. This was leading to a function not being
covered in the codebase. This change adds two new instruction sequences to
be recognised as an instance of the erratum, one with a load exclusive the
other with a store exclusive.

Differential Revision: https://reviews.llvm.org/D48840

llvm-svn: 336181