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

[Jack Styles via llvm-commits]

Stylie777 wrote:

@jayfoad I can only speak for Arm and AArch64 but the implementation we use to not reuse registers, unlike what is done in AMDGPU as you described. 

I think it would be easier here to just ensure AMDGPU is disabled, by keeping the `SupportsInitUndef` function and adding an implementation for AArch64. There does not seem to be any other architectures affected by this change.

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