ret guard makes the LLVM verifier sad

[jruderman]

fn main() {
    alt 2 {
      x when (ret) { x; }
    }
}

LLVM dies saying:

PHINode should have one entry for each predecessor of its parent basic block!
  %6 = phi i32* [ %3, %9 ]
Broken module found, compilation aborted!
Stack dump:
0.      Running pass 'Function Pass Manager' on module 'rust_out'.
1.      Running pass 'Module Verifier' on function '@_ZN4main17_58aa485ede601b15E'

with the stack:

0   librustllvm.dylib               0x0161ca67 abort + 39
1   librustllvm.dylib               0x015b8563 (anonymous namespace)::Verifier::abortIfBroken() + 211
2   librustllvm.dylib               0x015c9fc7 (anonymous namespace)::Verifier::runOnFunction(llvm::Function&) + 1927
3   librustllvm.dylib               0x01599f1f llvm::FPPassManager::runOnFunction(llvm::Function&) + 655
4   librustllvm.dylib               0x0159a02c llvm::FPPassManager::runOnModule(llvm::Module&) + 156
5   librustllvm.dylib               0x0159986f llvm::MPPassManager::runOnModule(llvm::Module&) + 639
6   librustllvm.dylib               0x01599b4c llvm::PassManagerImpl::run(llvm::Module&) + 172
7   librustllvm.dylib               0x01599c8b llvm::PassManager::run(llvm::Module&) + 27
8   librustllvm.dylib               0x009fc5a0 LLVMRustWriteOutputFile + 768 (RustWrapper.cpp:96)
9   rustc                           0x00009007 _ZN3lib4llvm4llvmE681 + 119
10  rustc                           0x00632a86 back::link::write::run_passes::anon7290::anon7291 + 150
11  libstd.dylib                    0x00908cc6 str::as_buf + 294
12  rustc                           0x006329cb back::link::write::run_passes::anon7290 + 187
13  libstd.dylib                    0x00908cc6 str::as_buf + 294
14  rustc                           0x004e3d77 back::link::write::run_passes + 14391
15  rustc                           0x0064106e driver::rustc::compile_input::thunk8003 + 158
16  rustc                           0x00557ea0 driver::rustc::time + 352
17  rustc                           0x0055f3c7 driver::rustc::compile_input + 23431
18  rustc                           0x0057ddb8 driver::rustc::main + 45960
19  rustc                           0x0057f90f _rust_main + 63
20  rustc                           0x00645b1f _rust_main_wrap + 31
21  librustrt.dylib                 0x0088af4c task_start_wrapper + 60 (rust_task.cpp:149)