[AMDGPU] SelectionDAG divergence tracking should heed AMDGPUSubtarget::isSingleLaneExecution

[jayfoad]

If a kernel is known to be executing only a single lane, IR UniformityAnalysis will take note of that (via GCNTTIImpl::hasBranchDivergence) and report that all values are uniform. SelectionDAG's built-in divergence tracking should do the same.

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Jay Foad (jayfoad)

If a kernel is known to be executing only a single lane, IR UniformityAnalysis will take note of that (via GCNTTIImpl::hasBranchDivergence) and report that all values are uniform. SelectionDAG's built-in divergence tracking should do the same.

[alex-t]

#144947