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

[Petar Avramovic via llvm-commits]

petar-avramovic 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

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.





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