Fix a crash in the lifetime safety dataflow analysis when handling null CFG blocks.
Added a null check for adjacent blocks in the dataflow analysis algorithm to prevent dereferencing null pointers. This occurs when processing CFG blocks with unreachable successors or predecessors.
Original crash: https://compiler-explorer.com/z/qfzfqG5vM
Fixes https://github.com/llvm/llvm-project/issues/150095