Bug in UCS while loop that merge else branch

[AnsonYeung]

Original code:

while f() is
  A() do ()
  B(xxx) do g(xxx)

When f return A(), the while loop terminates as the A() branch was merged with the else branch.

Full example to demonstrate that changing else branch affect the loop behaviour:

let x = 1
while x ===
  1 do
    set x = 2
  2 do
    set x = 3
    print("valid")
else
  set x = 2
print("not inf loop")
//│ > not inf loop
//│ x = 2

let x = 1
while x ===
  1 do
    set x = 2
  2 do
    set x = 3
    print("valid")
else
  set x = 3
print("not inf loop")
//│ > valid
//│ > not inf loop
//│ x = 3