Started occurring after f9d4149 merged

Repo exemplifying segfault

Relevant file in repo

To exemplify:

1. set LLVM_CONFIG environment variable to llvm-config path
2. cargo build --verbose

Result of stepping through the code with LLDB

Another example of a module pass exemplifying a crash:

bool InstrumentationPass::runOnModule(Module &M) {
  LLVMContext &C = M.getContext();
  IntegerType *Int8Ty  = IntegerType::getInt8Ty(C);
  llvm::Type *i = PointerType::get(Int8Ty, 0);  // segfault happens here
  return true;
}

This is preventing afl.rs from upgrading compatibility with any Rust version >1.2:

• SIGILL while compiling a project with afl.rs rust-fuzz/afl.rs#11