[llvm] [AArch64] Use an unknown size for memcpy ops with non-constant sizes. (PR #187445)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

LGTM too. Checked that this fixes a problem with an internal example. Would be a possible candidate for cherry-picking to the 22 release.

https://github.com/llvm/llvm-project/pull/187445