CFG: force the creation of a predecessor entry for all basic block.
CFG: force the creation of a predecessor entry for all basic block.

This ensure that all basic blocks in a function have a valid entry the CFG object.

The entry block has no predecessors but remains a valid basic block
for which we might want to query the number of predecessors.
Some unreachable basic blocks may not have predecessors as well.

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