[llvm] [AArch64] Use GISel for optnone functions (PR #174746)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I think the core change looks better now, but I don't understand the other pieces and they should be spilt out. This patch should not be touching the dominator tree 

https://github.com/llvm/llvm-project/pull/174746