Lower @llvm.experimental.deoptimize as a noreturn call

While preserving the return value for @llvm.experimental.deoptimize at
the IR level is useful during mid-level optimization, doing so at the
machine instruction level requires generating some extra code and a
return that is non-ideal.  This change has LLVM lower

```
  %val = call @llvm.experimental.deoptimize
  ret %val
```

to effectively

```
  call @__llvm_deoptimize()
  unreachable
```

instead.

llvm-svn: 265502