Issue Description:

... "Unknown access path instruction", line 139 in computeSubelement().

It can be triggered by compiling the following code with -O:

func foo<T>(_ t: T?) -> T? {
  let v = t!
  bar(v)
  return v
}

func bar<T>(_: T?) { }

_ = foo(42)

The SIL instruction that triggers it is the second init_enum_data_addr instruction (for preparing the return value).

There are other assertions in the file (saying that enum projections aren't supported) testing against the error value for computeSubelement()–except for the one codepath that triggers this. That, and the fact that the generated code works (with assertions off) leads me to think that maybe this one assertion just needs to be removed to fix this. I don't know enough about SIL to know if this is a sign of something going wrong elsewhere or what. I'd be willing to look into it some more if someone could nudge me in the right direction.