[AArch64] Clean up LSE directive tests

These were specifying an architecture version with .cpu directive,
which is invalid. As the error for this case outputs the problem
instruction we were still matching the expectations of FileCheck.

This patch fixes up the LSE tests to do what they seem to intend. A
follow-up patch will tighten up the directive tests.

Differential Revision: https://reviews.llvm.org/D47872

llvm-svn: 335585