DeadBranchElim: Improve algorithm to only remove blocks with no predecessors
DeadBranchElim: Improve algorithm to only remove blocks with no predecessors

Must be careful not to remove blocks pointed at by unreachable blocks

antiagainst/SPIRV-Tools,LukasBanana/SPIRV-Tools,dneto0/SPIRV-Tools,dneto0/SPIRV-Tools,KhronosGroup/SPIRV-Tools,LukasBanana/SPIRV-Tools,KhronosGroup/SPIRV-Tools,dneto0/SPIRV-Tools,KhronosGroup/SPIRV-Tools,greg-lunarg/SPIRV-Tools,pierremoreau/SPIRV-Tools,dneto0/SPIRV-Tools,pierremoreau/SPIRV-Tools,pierremoreau/SPIRV-Tools,greg-lunarg/SPIRV-Tools,KhronosGroup/SPIRV-Tools,antiagainst/SPIRV-Tools,dneto0/SPIRV-Tools,dneto0/SPIRV-Tools,pierremoreau/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,antiagainst/SPIRV-Tools,LukasBanana/SPIRV-Tools,KhronosGroup/SPIRV-Tools,antiagainst/SPIRV-Tools,pierremoreau/SPIRV-Tools,pierremoreau/SPIRV-Tools,KhronosGroup/SPIRV-Tools,pierremoreau/SPIRV-Tools,greg-lunarg/SPIRV-Tools,pierremoreau/SPIRV-Tools,dneto0/SPIRV-Tools,dneto0/SPIRV-Tools,antiagainst/SPIRV-Tools