Exclude variables' final types inside while true if re-assigned before first break

[HertzDevil]

Consider the following:

while true
  x = 1    # (1)
  if ...
    break
  end
  x = ""   # (2)
end
typeof(x)  # => (Int32 | String)

typeof(x) after the exit of the while expression should not be affected by the assignment on (2), because (1) must have been executed before any break could happen. This PR makes it so that typeof(x) becomes Int32.

Note that if (1)'s RHS references x itself then the exit type would still depend on (2) as well as whatever type x has before the whole loop:

x = {1}
while true
  x = x[0]  # (1)
  if ...
    break
  end
  x = {"a"} # (2)
end

# before this PR: (Int32 | String | Tuple(String))
typeof(x) # => (Int32 | String)

---

This issue was discovered while trying to determine whether while exp; ...; end is always semantically equivalent to the following, provided exp isn't already the true literal:

while true
  unless exp
    break
  end
  ...
end

while loops would be a lot easier to reason about if all of them could be cast like this. This flow typing issue turned out to be a blocker whenever the while condition contains an assignment (including #10350).

[asterite]

Thank you! ❤️