[llvm] AMDGPU/GlobalISel: Disable LCSSA pass (PR #124297)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Can you please put a minimal example in the PR description where LCSSA breaks things for the temporal divergence lowering, and explain why divergence lowering can't handle it?

It's not acceptable for the lowering to not handle it. Not-LCSSA is not a guaranteed property 

https://github.com/llvm/llvm-project/pull/124297