[AMDGPU] Unify divergent nodes if the PostDom tree has one root

This patch allows AMDGPUUnifyDivergenceExitNodes pass
to transform a function whose PDT has exactly one root
and ends in a branch instruction. Fixes
https://github.com/llvm/llvm-project/issues/58861.

Reviewed By: ruiling, arsenm

Differential Revision: https://reviews.llvm.org/D139780