Bad code generation due to underscore

[Theodus]

The following code causes invalid LLVM IR to be generated:

actor Main
  new create(env: Env) =>
    if false then
      None
    else
      _
    end

~/src/theodus/stuff/wat/issue-_ $ ponyc -d --verify
Building builtin -> /usr/local/lib/pony/0.15.0-321b7c6b/packages/builtin
Building . -> /home/theodus/src/theodus/stuff/wat/issue-_
Generating
 Reachability
 Selector painting
 Data prototypes
 Data types
 Function prototypes
 Functions
 Descriptors
Verifying
Basic Block in function 'Main_tag_create_oo' does not have terminator!
label %if_else
Error:
Module verification failed: Basic Block in function 'Main_tag_create_oo' does not have terminator!
label %if_else

[Theodus]

Here is an lldb backtrace an a debug build of the compiler in case it's useful:

~/src/theodus/stuff/wat/issue-_ $ lldb ~/src/ponylang/ponyc/build/debug/ponyc
(lldb) target create "/home/theodus/src/ponylang/ponyc/build/debug/ponyc"
Current executable set to '/home/theodus/src/ponylang/ponyc/build/debug/ponyc' (x86_64).
(lldb) run -d
Process 32460 launched: '/home/theodus/src/ponylang/ponyc/build/debug/ponyc' (x86_64)
Building builtin -> /home/theodus/src/ponylang/ponyc/packages/builtin
Building . -> /home/theodus/src/theodus/stuff/wat/issue-_
Generating
 Reachability
 Selector painting
 Data prototypes
 Data types
 Function prototypes
 Functions
 Descriptors
Writing ./issue-_.o
Process 32460 stopped
* thread #1, name = 'ponyc', stop reason = signal SIGSEGV: invalid address (fault address: 0x18)
    frame #0: 0x00007fffe8bd610b libLLVMCodeGen.so.3.9`___lldb_unnamed_symbol372$$libLLVMCodeGen.so.3.9 + 315
libLLVMCodeGen.so.3.9`___lldb_unnamed_symbol372$$libLLVMCodeGen.so.3.9:
->  0x7fffe8bd610b <+315>: cmpb   $0x1e, 0x18(%rbx)
    0x7fffe8bd610f <+319>: cmovneq %r12, %rbx
    0x7fffe8bd6113 <+323>: testq  %rbx, %rbx
    0x7fffe8bd6116 <+326>: je     0x7fffe8bd612d            ; <+349>
(lldb) bt
* thread #1, name = 'ponyc', stop reason = signal SIGSEGV: invalid address (fault address: 0x18)
  * frame #0: 0x00007fffe8bd610b libLLVMCodeGen.so.3.9`___lldb_unnamed_symbol372$$libLLVMCodeGen.so.3.9 + 315
    frame #1: 0x00007fffe6d8184f libLLVMCore.so.3.9`llvm::FPPassManager::runOnFunction(llvm::Function&) + 559
    frame #2: 0x00007fffe6d81a3b libLLVMCore.so.3.9`llvm::FPPassManager::runOnModule(llvm::Module&) + 43
    frame #3: 0x00007fffe6d81e27 libLLVMCore.so.3.9`llvm::legacy::PassManagerImpl::run(llvm::Module&) + 743
    frame #4: 0x00007fffe890fca7 libLLVMTarget.so.3.9`___lldb_unnamed_symbol24$$libLLVMTarget.so.3.9 + 327
    frame #5: 0x00007fffe890faf1 libLLVMTarget.so.3.9`LLVMTargetMachineEmitToFile + 161
    frame #6: ponyc`genobj(c=0x00007fffffffd8b0) at genobj.c:71
    frame #7: ponyc`genexe(c=0x00007fffffffd8b0, program=0x00007fffe4ddfd00) at genexe.c:437
    frame #8: ponyc`codegen(program=0x00007fffe4ddfd00, opt=0x00007fffffffddb8) at codegen.c:1028
    frame #9: ponyc`generate_passes(program=0x00007fffe4ddfd00, options=0x00007fffffffddb8) at pass.c:301
    frame #10: ponyc`compile_package(path=".", opt=0x00007fffffffddb8, print_program_ast=false, print_package_ast=false) at main.c:249
    frame #11: ponyc`main(argc=1, argv=0x00007fffffffdf28) at main.c:374
    frame #12: 0x00007fffe54d66e5 libc.so.6`__libc_start_main + 245
    frame #13: ponyc`_start at start.S:118