Fix exit types of variables assigned inside while conditions

[HertzDevil]

Fixes #10350.

a = 1
while ((b = 1); a)
  a = nil
  b = "hello"
end
typeof(b) # => Int32

def foo(x, y)
  rand < 0.5 ? 1 : nil
end

while foo(b = 1, a = 1)
  a = nil
  b = "hello"
end
typeof(b) # => Int32

It turns out get_while_cond_assign_target isn't needed at all; if a while condition declares multiple variables, all these variables outside the loop can receive their types in the conditions, except that if a variable is nil-if-read (i.e. it is not initialized in all execution paths of the condition expression), then it also receives the type at the end of the while body because the assignments in the condition might not be run after a loop iteration:

while rand < 0.5 ? (x = 1; rand < 0.5) : false
  x = 'a'
end

# `x` has the type of `1`
# since `x = 1` might not be run and `x` does not exist before the loop,
# `x` has also the type of `'a'` and becomes nilable
typeof(x) # => (Char | Int32 | Nil)
# before: # => (Char | Nil)

One known issue is simply reading the variable in the loop body resets the nil-if-read status of that variable (in Crystal::MainVisitor#visit(node : Var)):

while rand < 0.5 ? (x = 1; rand < 0.5) : false
  x # variable read
  x = 'a'
end

typeof(x) # => (Int32 | Nil)
# before: # => (Char | Nil)

This is added as a pending test case. A workaround is to define x before the while expression, so that it is never nil-if-read. (I suspect the reason this doesn't happen to if expressions is because they have before_then_vars whereas while loops have no such equivalent.) This does not impact #10350.

The second commit fixes that.

[asterite]

Awesome!