Clean up IndirectControlFlowAnalyzer::FindJunkInstruction

1. Don't use an instruction adapter
2. Just call Instruction::getControlFlowTarget
3. Call it on all architectures
4. The check for x86 0x0000 is a hack to find zero padding compilers sometimes add at the end of functions for alignment. It is a valid instruction `add byte ptr ds:[rax], al`, but it's assumed to be padding.