[NFC][AArch64][CodeGen] Add fixed-width vector tests for get.active.lane.mask

In practice we don't expect to see the get.active.lane.mask intrinsic
being used for fixed-width vectors, but we should at least be able
to generate code for it. This patch simply adds some fixed-width tests
to an existing file:

  CodeGen/AArch64/active_lane_mask.ll

Differential Revision: https://reviews.llvm.org/D116644