[llvm] [AArch64] Extend and rewrite load zero and load undef patterns (PR #108185)

[David Green via llvm-commits]

davemgreen wrote:

Hi. It shouldn't need mir tests, but eventually will need some test coverage for GISel. It is better to test at the .ll file level where we can. GISel still needs more work for this to trigger though, this patch is currently just targeting SDAG.

https://github.com/llvm/llvm-project/pull/108185