[llvm] [InitUndef] Enable the InitUndef pass on AArch64 (PR #108353)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Though I'm wondering if it wouldn't be better to enable it by default and only disable on AMDGPU, as that is the conservatively correct behavior, and it doesn't seem like this affects register allocation/scheduling on other targets anyway.

Yes, the default should always be conservatively correct. This type of stuff is very undiscoverable 

https://github.com/llvm/llvm-project/pull/108353