Compiler crash on exhaustive match where last case is a union subset.

[Perelandric]

(Maybe related to #2028 ?)

Given this start of a program that uses a Tester class:

use "debug"

actor Main
  new create(env: Env) => Tester.test()
    
primitive Foo
primitive Bar

type Foobar is (Foo | Bar)

The following two definitions of Tester segfault the compiler:

https://is.gd/jgPLZL

class Tester
  fun test() =>
    let part: (Foobar |  None) = None

    match part
    | None => Debug("none")
    | let c': Foobar => Debug("Foobar")
    end

---

https://is.gd/cgfwRR

class Tester
  fun test() =>
    let part: ((Foobar, Foo) |  None) = None

    match part
    | None => Debug("none")
    | (let c': Foobar, let ss': Foo) => Debug("Foobar")
    end

---

EDIT: The error for this one is fixed on master via #2002, so that makes this identical to the one above.

And this one resulted in a compile-time error with the 0.15.0 tagged release, but segfaults the compiler on the master branch:

https://is.gd/DUdo1i

class Tester
  fun test() =>
    let part: (Foobar |  None) = None

    match part
    | None => Debug("none")
    | (let c': Foobar) => Debug("Foobar") // Note the parens around the `let` declaration
    end

[jemc]

This turns out to be a pretty simple fix once I found the problematic code. You found an edge case I wasn't accounting for in the codegen for exhaustive match.