[AArch64] Do not use ABI alignment for mops.memset.tag

Pointer element types do not imply that the pointer is ABI aligned.
We should be using either an explicit align attribute here, or fall
back to an alignment of 1. This fixes a new element type access
introduced in D117764.

I don't think this makes any practical difference though, as the
lowering does not depend on alignment.

Differential Revision: https://reviews.llvm.org/D118681