Fix throw_undef_if_not error

[penelopeysm]

Closes #204, and adds a regression test.

The problem is that the SSA registers inside Expr(:throw_undef_if_not, ...) were not being referenced/dereferenced in the same way that other SSA registers are. The fix itself is a one-liner, it just applies the existing SSA register handling mechanism to throw_undef_if_not.

In the original reproducer in #204, this is the original function's IR.

2 1 ─ %1 = (_2 === 1)::Bool
  └──      goto #3 if not %1
  2 ─      nothing::Nothing
5 3 ┄ %4 = φ (#2 => true, #1 => false)::Bool
  │        $(Expr(:throw_undef_if_not, :y, :(%4)))::Any
  │        invoke Main.produce(3::Int64)::Int64
6 │        $(Expr(:throw_undef_if_not, :y, :(%4)))::Any
  │        invoke Main.produce(3::Int64)::Int64
7 └──      return 1

The first throw_undef_if_not is fortuitously handled because it references %4, which is derived from a PhiNode, and the PhiNode handling ensures that the value of %4 is indeed dereferenced correctly.

The second throw_undef_if_not would be fine if it ended up in the same CFG block as the previous one. However, because there's a produce statement between the two, it ends up in a different block, which leads to the error seen in #204.

(Claude helped me work through the codebase, but the PR and fix is mine.)

[github-actions]

Libtask.jl documentation for PR #216 is available at:
https://TuringLang.github.io/Libtask.jl/previews/PR216/

[yebai]

Nice improvement -- interesting to know that coding agents can help with understanding tricky code!