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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> GlobalISel temporal divergence lowering(next 2 patches) + new regbankselect(and regbank legalize) can handle both LCSSA and Not-LCSSA. This was discussed some time ago that globalisel path should disable LCSSA since it is not required for correctness. SDAG, the fix passes after isel that move-to-VALU, silently require LCSSA to deal with temporal divergence.

But that still breaks the fallback. This should be an optional step, and I'd prefer to do it after. It can also only do this in the case where the fallback is not enabled 

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