[llvm] [LV][AArch64][NFC]: Change TC in a test case. (PR #157512)

[David Sherwood via llvm-commits]

https://github.com/david-arm approved this pull request.

LGTM! I realised now that `@main_vf_vscale_x_16` already exists in sve-epilog-vect.ll for the default case where we prefer the fixed-width epilogue, so it seems fine to simply change this test to use a variable trip count.

https://github.com/llvm/llvm-project/pull/157512