[AArch64] Fix merge-store.ll regression test after r273271

r273271 changed the RUN line of the regression test to use
-march=cyclone instead of -mtriple=aarch64-none-none.

This caused a change in the output syntax for the ext
instruction, causing the test to fail. Change this test
back to using -mtriple=aarch64-none-none.

llvm-svn: 273286